The only way to get this marco to work even party way is this

/cast [target=party1target,harm,mod:shift][mod:shift] Frostbolt
/cast [target=party1target,harm][] Fireball

try this out using 2 mage agaist the target dummys and a few bats.
Both mage will cast fireball
when I hit shift, both instance will show the toggle to frostbolt, but only the 1st mage. will cast the frostbolt, the slave will said spell not ready. Tab over to the slave and hit shift+1 will cause slave to cast Frostbolt. ?(