I already know they shared the same universe through some mention of the Black Mesa organization - namely in the first Portal's Still Alive end song and the second game's Cave Johnson comment that Black Mesa was stealing their ideas.
I was surprised to notice the most complete Portal wikia is half-life.wikia - that leads me to think the link between both stories is bigger than some small Easter eggs.
Am I mistaken, or is there a bigger link between both storylines?
If you wander around the correct locations in Portal one you see there various slides from presentations that hint at a rivalry between the two companies, both of which were working on similar projects. As well as this, the dry dock for the Borealis is seen in Portal 2.
Teleportation was a major field of study at Black Mesa, and their portals were used primarily in the procurement of Xen crystals, as they were used to transport scientists to and from the Borderworld for research and analysis. ~Half Life Wiki (referencing game events)
Single dimensional teleportation is seen in the second chapter of Half-Life 2, Red Letter Day when the resistance tried to teleport Gordon Freeman to a separate base, and achieved by factoring in 'Dark Energy equations'. In effect the teleportation swings by Xen, and arrives elsewhere on earth
Conversely the teleportation technology developed by Aperture Science is based on portal technology and is in essence instantaneous. The supposed delay in the teleportation of Gordon is due to interference from the combine teleporter being destroyed, otherwise teleportation as developed by Black Mesa is also instantaneous.
Some time before the 7-Hour war (circa 1970, from Portal 2), it is found out that Aperture science is close to completing/advancing their portal technology, sans safety checks:
The Borealis is an Aperture Science research vessel introduced in Half-Life 2: Episode Two. According to Isaac Kleiner, Aperture was working on a promising project, but in their rush to beat Black Mesa for funding, they neglected ordinary safety rules and the ship simply disappeared with parts of its drydock, which earned it an almost legendary stature.
Years later, Judith Mossman, Resistance operative, finds the Borealis and sends a message to that effect to the resistance in White Forest.
I'm fairly sure I've pinned down the location of the Project. It's hard to say how much of it might have survived intact, or whether there's anything remaining that could compromise our work... if it were discovered by the Combine. We'll need to take a close look at it, of course, but I should be able to give a better opinion within a few hours. If the site is where we think it is, then it should be no more than... I'm going to cut this short. We may have been spotted.
The resistance decide from this and now decrypted photos and blue prints that whatever Aperture science developed and is now on the Borealis is either going to be a powerful weapon against the Combine, or too dangerous to be left in the hands of the combine. Either way it signals how the two companies and more importantly stories are intimately linked.
I wouldn't say they happen at the same time. The events of HL1 and HL2 are 20 years apart. At the beginning of P2 you're in a bed and wake up periodically, but something happens and you're stuck for more time than the clock can track (hence in not rolling over). It can be assumed that you've slept for way over 20 years. Somewhere during P2 GLaDOS states that you only have 60 (I think) years of your life left. Subtract sixty from one-hundred and you get forty. In the original game you were thirty. Ten year difference. Since you can basically live forever in stasis, the longest amount of time from Portal to Portal 2 is roughly 720 years- enough time for natural life to take over. I can also say that the Borealis is still out there and that it stayed on Earth, because the operative found it and in the art for (possibly) HLe3 the combine had found it. The Aperture Sciene tech on that ship does not transport through time and space- it goes against context for one thing. The Aperture Science hand-held portal device gives a clue- you don't just call something hand-held. Take a cup for example- you don't call it a handheld cup- there's nothing bigger than it. Looking at the blueprints for the Borealis on the Half-Life wiki it would be most probable that there is just a more advanced version of the portal gun. If it made bigger portals, then that would make sense for the Combine to want it. They could transport their people much more quickly for a full-scale invasion. Sorry to ramble, but yes, the two games are connected.
We will probably know more when the next Half-Life episode is released. In the end of Episode 2, they mention a strong connection between the Borealis (which will probably be a fundamental part of Episode 3) and Aperture Science. It is safe to speculate that we will learn more about the connection between both companies in the next instalment. So far we have only the connection you mentioned.
The link between the Portal series and Half-Life series is that both series are in the same world. As in that they have direct relation to each other.
In Portal 2, you see in an Easter Egg of the Borealis, from Half-Life 2 being built.
We probably will learn more in the next few games, as bitmask has pointed out. But right now you can see that the two companies were at an arms race of being the best. The plot is just unraveling.
Aperture Science is an underground science facility while Black Mesa on the surface. C. Johnson always mentioned about his investor. In Half Life, however, there were not a lot of people who were in a state to invest so we can say that the events of Portal 2 may be in parallel to Half Life, but at the time when Johnson was alive Black Mesa was just a competitor of Aperture Science.
From what I've read, GLaDOS was activated on the same day as the invasion of the Combine. Thus, due to the world being preoccupied, the fate of Aperture Laboratories was not noticed. Hence the reason nobody tried to save the test subjects and employees that GLaDOS most likely murdered.
The fact that Aperture Science created the Borealis and the Gravity Gun along with the Dollars And Sense: A Guide To Competing With Black Mesa slide show in the office areas in Portal 1 show that the connections are very great.
In the end this was just an excuse to use the same texture files in the games.
Citing the comic Lab Rat (by Valve), Aperture actually made the Gravity gun. It was then stolen by Black Mesa or their affiliates. It is presumed that the same thing happened with the Borealis because of mumblings by Doug Rattman that were slowed down. Aperture and Black Mesa were huge rivals.
I'm not going to say that I understand the exact connection between Aperture and Black Mesa, but what I do know is that a common theory is that Portal 2 is happening around the same time as Half Life 2, and as we all know at the end of Portal 2 Chell goes into the outside world. This may lead to further connections between the two games in the future. Just a theory.
