The focus macro to set up the focused targets for your alts:

/target focustarget (or /assist focustarget)

The spells you want to send to your focused targets (without using focus)

/cast [target=target] Spell (ie. /cast Spell, just added target=target for clarification.)

All your dps macro's that don't go on the focus target (But on the main char's targer):

/cast [target=focustarget] Spell

That should work fine I think.