Put 2 skills on your bar that are sure to cast ie. frost armour and make sure that keyclone is working.

Once youve got that set up try this simple macro

on your main
/cast Fireball

On your alt
1st macro
/focus (mainname)

Do that one on a seperate key( you could also set your focus by rightclicking their portrait and doing it manually)

then test your dps macro

/assist focus
/cast fireball

See how that goes.