Two things:

1) There is a lag of about .5 to 2 seconds from the time your main targets something new until the time your alts can see that your main has targeted something new. You just have to be patient and figure out what your average lag time is. Note: it will be different in instances than in the outside world.

2) Consider that, if you're trying to heal whoever the mob or enemy player is attacking, you can always use /cast [target=targettarget, help] Flash Heal, instead of trying to manually target and then heal. That way you just keep your enemy targeted and go to town.

This help any?