One of the drivers behind the 'Rule of Two' is that with fewer Sith, the dark side will become more concentrated in those users and they will be more powerful as a result. Is there any indication that the same is true for Jedi?
That's not how
the force works! or why the Rule of Two works. The Rule of two was created by Darth Bane to allow the Sith Order to operate in secret, because greed and fighting amongst many Sith Lords is what caused their downfall in the first place.
The Rule of Two isn't also about concentrating power. It is a cycle whereby the Master embodies power, and the Apprentice craves it.
So, since it isn't even true for Sith, it certainly isn't true for Jedi.
To the best of my knowledge, the idea that the Rule of Two "concentrates" the dark side into fewer Sith and makes them even more powerful is only mentioned in the Bane trilogy, which is Legends. Thus there is no evidence in canon that either the light or the dark side can be made more powerful if "concentrated" into fewer Jedi or Sith, respectively.
The Bane trilogy does explicitly mention Bane's belief that the Rule of Two served a secondary purpose of concentrating the dark side into fewer users and thus making it more powerful:
“The power of the dark side is strongest if it is concentrated in one vessel,” Bane replied. “Not spread out among many. I did it for the sake of the dark side.
Star Wars: Path of Destruction, p. 285
The main purpose of the Rule of Two was to stop the infighting among the Sith, though (emphasis added):
By its very nature, the dark side invites rivalry and strife. This is the greatest strength of the Sith: it culls the weak from our order.
The constant battling of the Sith since the beginning of recorded history served a necessary purpose: it kept the power of the dark side concentrated in a few powerful individuals. The Brotherhood had changed all that. There were now a hundred or more Dark Lords following Kaan, but most were weak and inferior. The Sith numbers were greater than they had ever been, yet they were still losing the war against the Jedi.
The power of the dark side cannot be dispersed among the masses. It must be concentrated in the few who are worthy of the honor.
The Jedi would always remain united in their cause. The Sith would always be brought low by infighting and betrayals.
Star Wars: Path of Destruction, p. 237
Note that all of these quotes indicate that it is in the nature of the dark side only to benefit from concentration in fewer individuals. Bane believed that the Sith infighting served to cull the weak and concentrate the power of the dark side in a few individuals. The Rule of Two was designed to stop the infighting (which would weaken the Sith as a whole against the Jedi), and the concentration of the dark side into two individuals was a beneficial side effect. The Jedi are "always united", though, so there is no concentration of the light side into a few individuals. Thus, in Legends, only the dark side seems to benefit from being concentrated into fewer individuals. I suppose it is possible that the light side benefits as well (in the rare event that the Jedi are mostly exterminated, for example) but we can't be sure because only Bane talks about the Force being concentrated and he was not familiar with the light side.
Not canon (yet...) but I have a theory that the Force is like 1 reservoir, and the Force sensitive people are like conduits from it. If you shut off (or kill) a huge number of the conduits, the pressure behind the operational ones greatly increases. The thing that made me think this besides the rule of two with the Sith, is now we have Kylo Ren who can hold blaster shots in mid air, nobody has ever done that before. Why? I think with a galaxy full of force users the Force was merely a trickle and now when it has an outlet it's a roar.
Again, just a theory.
So, it sounds like Bane is saying there is a finite about of dark force within a local area. If 100 users were able to tap into that with 100% efficiency, they would each have 1/100th of the power, perhaps sufficient to hold their own against a jedi. BUT if only 2 were there with 50% of the available power, they would be a 'force' (pun intended) to be reckoned with. Not only would they be more powerful, but they would also instill more fear in their opponent, which they can use to even greater advantage. Not to mention, it would be easier to be covert, less costly, able to fight on more fronts, and lower chance of intelligence loss. Suffice it to say, their enemies, namely the jedi were more concerned about a couple powerful Sith than 1000 wannabes.