The answer to this one is yes and no. Multi-player exp calculations are a little complex.
First up, each monster's base exp is scaled up by a factor of 4.5 in a full game. This is totally irelevant of where the 8 players are. The monsters walk around with this number, ie "I am worth this many exp".
When you kill one though, the game checks your party situation. If you are not in a party,
or if nobody in your party is close by, you just get the base exp, 4.5 factor included, the same as anyone else in that game playing by themselves.
However, if you are in a party
and close by eachother (both must be true) then you are included in a further calculation. The game takes the base exp and adds an additional 35% for every close-by party member. So if you have just one buddy nearby, you get 35% more exp thrown in. If all 8 of you are partied and packed into the Throne Room, everything is worth an additional 7x35=245% more.
However, then you have to split that amongst all the nearby party members. The split is done based on your fraction of the total levels, so you get less if you are lower level. But assuming you are all the same level, in the 8 players in the Throne Room case you'd end up getting 43% of the monsters' base exp each. Of course, that is 43% of the 4.5x they are worth in a solo game, so you still end up with nearly twice the value of a monster in a solo game.
Going back to the original question:
For example, let's say that there are 8 people in a game and 4 people are doing a chaos sanctuary run and the other 4 are doing a baal run. Would that be more exp for the baal runners than if all 8 people were in the baal run?
If all 8 are doing a Baal run (and assuming they are all the same level), each will get about 1.94 times each monster's base exp in a solo game.
If 4 are doing Baal and 4 are doing CS, each will get about 2.3 times each monster's base exp in a solo game.
So, from that perspective, you will earn slightly more in the latter case. However, bear in mind that in both cases, the monsters will all have 4.5 times their base HP in a solo game, but in the first case you have 8 times the total offensive power of a solo player and in the second only 4 times. So the time spent per exp (which is what you really care about) is not so easy to determine.