This may turn out to be a stupid question, but....

Is it possible to use Multiple Attacks against more than one foe you're engaged in melee with? For example, if I have an Assault Marine with Swift/Lightning Attack, and am in melee with more than one opponent, can I delegate one or two of my attacks to a secondary target? Can I delegate all four attacks to different targets? (Assuming using two weapons)

yes as long as they are in range of your attacks. you should declare a target then attempt to hi him, then declare a target, and attempt to hit him.. you should not roll all your attacks then declare targets.