When my main character casts Shadow Bolt, my second character runs a macro:

/assist focus
/cast Shadow Bolt

My main is the focus obviously, so on EVERY spell cast my second character reaqquires the target. I only ever find this problematic when dealing with /petattack.

I realise you are dealing with two different classes here, so I don't know how well this solution applies to you.

As for /follow, there are specific things that you can and can't do while maintaining follow. My experience is only with Warlocks, but it should be sufficient to give you a good idea:

All instant casts, and spells with cast time do not break follow, unless your main runs out of follow range while the alt is standing still casting.
Autoattack breaks follow.
Spells that auto-face, like the Drain Soul/Life/Mana or Health Funnel will break follow.

That twoboxtoolkit addon mentioned above is brilliant. I don't use the autofollow function, as it requires you to have one character to be the designated master, and my setup doesn't go like that, but you may find it to your liking.