One word of advice for boxing a caster of any kind, macro macro macro. Make some of the casters work macro'd, for instance, if you want to box a shaman, make a macro that has malo/slow, that way you can focus on your other char for longer then if you are casting both spells individually. I boxed an sk/shm in live to 65/100aas in the PoP era, as far as live goes, this was a killer combo. Single pulls from sk made it so I never had to deal with CC and could just malo/slow/dot then sit until sk needed heals. as for a healing macro, depending on the ratio of your heal vs. other chars total hp, make a macro that heals several times in a row, and just wait for your hp to drop to 50% or so then hit the macro and go back to focusing on your melee char.
On SoD i decided on bst/druid combo. It's much more sloppy then sk/shm, but also more fun because of it. If I can get a single pull, I usually use a dot/nuke macro on the druid and stay on the bst during that time. If i don't get a single pull, I just grab aggro with bst/bst pet then wait until one or both needs healing and do so accordingly. You'll find that aggro management makes boxing so so so much easier. If your caster has hide, like my druid does, make a macro that does /sit then /doability X, where x is hide. That way you have a chance of not being raped by an extra mob during pulls. I usually also put an /afk talk to XXX, this toon is a box, so I am only using one character to communicate, its easy to get confused who your talking to on what character if you used both to communicate.
But it does just take practice to get good with it. When I started playing SoD after about a 4 year absence from EQ, I was very sloppy and felt totally unorganized. Now at about lvl 30 I'm fairly proficient at it. Just stick with it and you'll get used to it.