/assist <charName> causes you to target the current target of <charName>.

So your main targets a mob, your alt hits his /assist main button and now your alt is targeting the mob as well.

/focus <targetName> sets a secondary target that is remembered until you set a new focus and is accesable as a target for your macros. ie. [target=focus]

So your alt sets /focus main for example and now any macro where you have the modifier [target=focus] will cast at your main until you set a different focus.

http://www.wowwiki.com/Focus_target