I call it "the Wardrobe Paradox." Would they arrive independently at their respective time of entry, or would one of them arrive at the other's time?
The only rule when it comes to time spent in Narnia is that there is no rule about time spent in Narnia. Lewis intentionally made things inconstant, arbitrary, and dare I say, magical.
That said, Lucy enters the wardrobe to visit Mr. Tumnus at one point, and then Edmund comes in a few moments later. They leave the wardrobe together, and end up at some point after Edmund comes in. So in your example, they'd probably exit in the second person's time.
The comparison between Narnian time and time in our world isn't consistent, except in one respect: time flows in the same direction in both worlds. A year into the future in our world might be three years or three thousand years into the future in Narnia, but it won't be a year into the past.
So if person A enters Narnia from our world at one point (1920, say), and then person B enters ten years later (in 1930), then a certain amount of time will have passed in Narnia during those ten years in our world. If that time is less than a human lifespan - say, twenty years or so - then A will still be alive, albeit older, when B enters, so it will be possible for them to return together. The point at which they do so, in our world, will be after B left this world for Narnia, but we can't say more than that.
The fact that all journeys into Narnia seem to last only moments in our world seems to be a fluke - or, more likely, the will of Aslan. It's clearly impossible that both A and B could re-enter our world only moments after they left, but it would be possible for B to do so. The most likely answer for their time of return is probably moments after B left.
To put it another way, Peter entered Narnia in Prince Caspian approximately a year before Eustace entered it in The Voyage of the Dawn Treader. If Peter had stayed there all that time (three years, by Narnian time), then he would still be there, perhaps together with Caspian to greet Eustace, and the two of them could then have gone back to our world together as Edmund, Lucy, and Eustace did at the end of The Voyage of the Dawn Treader.
At the end of Prince Caspian (the book, that is), Aslan creates a door in the air. Peter, Susan, Edmund, Lucy and the Telmarines who are going to our world go through the door in single file, each successive one's hands on the previous one's shoulders (presumably the first Telmarine had to stoop to rest his/her hands on Lucy's shoulders unless he/she was quite young or very short!). As the Pevensies pass through, they see three things at once: a glade in Narnia, a South Sea island in our world and a railway station in England. So the portal has two simultaneous exit points in our world, and each person is directed to the right one for him or her. While it may be assumed that both points exist at the same point in our world's time, we don't know that for certain. Suppose the pirates who became the Telmarines' ancestors left our world in 1750 (long before Digory and Polly in The Magician's Nephew were born, so predating the creation of the Narnian world apparently, but possible if the arrow of time in the Narnian world is allowed to go backwards with respect to ours on occasion). Then, when Aslan tells them that the original race of pirates has died out, it could be 1850, while the Pevensies arrive back in 1942 or whenever. Since Aslan says that the island hasn't been discovered by the rest of our world yet, that's far less likely in the mid-20th Century when we have air travel and satellites are only 20 years away.
So my view is that, as the whole point of arriving back just moments after leaving is not to have anyone missing from our world for any noticeable length of time, person A would arrive back 10 years before person B.
As Aslan is God among beasts and man, the answer is simple: Whatever time he wants them to go back to. Whether it be both the earlier time, both the later time, both their own time, or any darn time imaginable. He is a wild lion, unrestrained by your puny sense of logic.
It can be read that the Telmarine (the people Prince Caspian leads and is from), came to Narnia after the Pevensies but are possibly from a time before them. So linear consistency may not be a factor in travelling to and fro from Narnia.
Elaborating on the previous answers...
Towards the end of his life, Lewis had started a work that would tie his Narnia and Space Trilogy sets together called "The Dark Tower" (no relation to the Stephen King set by the same name). Unfortunately, the work was never completed.
It starts with the premise that time is three dimensional, that there are ways to measure it in all dimensions, that travel between timelines is possible, and that the relationships between timelines (worlds in the parlance of Narnia) are quite variable.
@Lan has the right of it with this additonal information on the nature of time in Lewis' universe.
I think that if they come back together they will both go back to their own times. Otherwise, if they both went back to the same time, say, the second persons time, what would happen to the first persons time? Lucy went into Narnia alone and came back alone and it was like no time had passed at all. So, back to the two people, if they both went back to the second persons time the first persons time would just have to "stand still" forever.