Just my 2 cents. Help and harm check for exists as I understand it. So you don't need to put exists in 90% of the time. The next part that is just me, I don't want my alts attacking anything other than what my main has (I do have one pet attack button that will send the pet at the mob that the alt has selected so I can split up mobs with my pet classes to share damage). So I only use the [target=focus-target] part. If that part isn't true, the alt stands and waits. I have almost never had a problem switching or aquiring targets with my alts. My guess is your alt doesn't see that your main has a target and attacks the one they have. You can see your focus either with a good UI mod (I use x-perl), but there are focus-frame mods that will show you what you have focused. If you use that, you can see what mob your alt sees its focus has selected. Just pick a really crappy mob (so you don't get hurt), select the mob on your main and look at your alt's screen. Without pressing anything, you should see what mob the main has selected (on your focus frame). Then push your macro and see what mob the alt selects. Just remember, changing targets takes time. You select the mob, it goes back to Blizzard (add lag time), then it goes back to your alt (add lag time) and then your alts sees it.
Hope that helps and good luck.