What does it matter if they are connected or not?
In no real fight for a bijuu or whatnot will your opponent allow you to go into the same dimension as theirs. That altogether defeats the purpose of it. You use it to escape. If you both enter the same realm then they could theoretically say since they're in the same area in the real world that they're at the same distance in the alternate dimension thus even if you tried to kamui through an attack, they would mimic the kamui and cut you in said other dimension.
Then we have to get in the debate of how big that dimension is and if the two could ever reach one another. Is it millions of miles big or just the size of the world? Could you sense others in that dimension? Etc.
Seems logical then, that if you are in the same relative area in the real world, then you are close to the same relative area in the Kamui world. Kakashi vs Obito for support of that concept. You can likely sense the others within the dimension as well for all practical sake.
In a real fight, even if you try to flee, it is possible to (with both eyes) sync with the other Kamui dimension user and still get into ther dimension. There is only so much that you can control in the RP, and that is technically a very valid tactic for countering an opponent with Kamui/a space-time ninjutsu like Kamui.
The purpose of any technique is to defeat your foe or give the user an advantage of some sort. Your foe naturally is going to do anything that they can to counter that. Therefore, the significance of the debate is that each Kamui user having their own pocket dimension has simply allowed Kamui to be more abused than if the Kamui users were all connected to the same dimension, despite the latter being a bit more logical in a 'verse where there are multiple Kamui users.
Do hiraishin users all use the same dimensional void when they temporarily make their jump? It's likely, but due to the complexity of the technique formula, the nature of the technique that you can only warp to a location that you marked, and the fact that the user is within the dimensional void for such a miniscule amount of time, the user cannot just warp to other people's hiraishin markers without syncing or pirating their marks.
With Kamui, you can warp virtually everhwere within a dimension of your "mastery" so's to speak. By that logic, the Kamui dimension you can warp to at will, giving an advantage to having both eyes as Kamui than just having one as Kamui when in race to out-Kamui your opponent.