i use this method you just described
and will use it for my shamans too when they get hex

it works quite nice, /focus main never fails, it is always correct
/target when used with the right [notes] works like a charm

my spells would be
/cast [target=target,harm,nodead][target=focustarget,harm,nodead]spell


ONLY WARNING ABOUT THIS !!!
this does not work nice with hunters
i used this system to mark different targets for different pets
ie. F1 does
/assist focus
/cast huntersmark
etc etc

when pressing my attack pet macro before combat it works:
/cast [target=target,harm,nodead][target=focustarget] petattack

but the problem with hunters as SOOON as they fire they're first shot with my cast macro
/cast [target=focustarget] !Autoshoot
the hunters would change theyre target to the one they are casting
and any subsequent use of the /target system fails


as far as i know this only applies to hunters, and not to normal casters
since a normal caster does not change its target while using the /cast [target=focustarget] system