63 damage done 3.5 at a time is on the order of two minutes' worth of rounds. Assuming that you are starting this after initiative has ended, why is this a problem? Taking two minutes to properly sear the wounds of the troll to be acceptable, especially considering how long it takes to sear a steak.

Additionally, if intelligent players sever the limbs first, even if the main troll body regenerates enough to wake up, it won't have any way to actually attack.

According to the full-attack description on the d20 SRD:

If you get more than one attack per round because your base attack bonus is high enough, because you fight with two weapons or a double weapon or for some special reason you must use a full-round action to get your additional attacks. You do not need to specify the targets of your attacks ahead of time. You can see how the earlier attacks turn out before assigning the later ones.

The fact that it specifies that you can decide who to target for each attack after the previous attacks have been resolved means that you can target different creatures with each attack. This also explicitly allows attacking a second creature if you've killed the first one.

Note that you must still use your attacks in order from highest-to-lowest attack bonus:

If you get multiple attacks because your base attack bonus is high enough, you must make the attacks in order from highest bonus to lowest. If you are using two weapons, you can strike with either weapon first. If you are using a double weapon, you can strike with either part of the weapon first.