I've just begun my first foray into BGII: SoA. Before I started I posted a question about whether to play it solely as SoA, then when completed install ToB ... OR ... load up SoA and ToB at the same time.

The concensus seemed to be load them at the same time so I did.

Does ToB play a smiliar role to SoA as ToTSC did for BGI? Meaning, can ToB be completed before ending SoA or does its main plot come after?

Also, I assume loading ToB boosts the XP cap?

No. ToB is different. It is a continuation of the stroy. So you finish SoA first, then only you go to ToB. Once you get to ToB, you cannot go back to SoA.

There is only one area that is an exception that you can play aytime during SoA or ToB. That is the Watcher's Keep. However, it is recommended that this area be played during ToB (at the beginning of ToB), after finishing SoA.