Ok, so in our DnD 3.5 campaign we're using a custom setting with reduced magic but includes early firearms e.g. flintlocks, revolvers, muskets, Dwarven Hand Cannon (prototype shotgun). My character has Two-Weapon Fighting and has a pair of revolvers (with attached blades) in his arsenal. As far as I can tell I can only use two weapons against the same target in one attack using Two-Weapon Fighting; what sort of feats would I need to use each one against a different target in one round? (for rules purposes, we're more or less treating firearms with the same rules as crossbows/hand crossbows/repeaters).
There is no rule stating that all attacks made during a Full Attack action need to be against a single target. This is true regardless of why you are making multiple attacks. In fact, as long as you declare you are making a full attack action, you can switch targets after ever attack, based on if it hit or not, etc.
This is backed up by the rules for Full Attack actions which states:
If you get more than one attack per round [...] because you fight with two weapons [...] 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.
Note that this is only applicable to Full Attack actions. Any other action allowing multiple attacks, (such as maneuvers or feats like Cleave,) will impose their own restrictions on if you can or cannot switch targets.