I spent just a little while tonight using my 1st and 2nd account mains together (73 lock and 68 hunter), but wasn't doing anything serious, just killing a bunch of basillisks round shattrath to get scryers rep.

Both classes take a lot of spell bar space, and you use all those spells together, weaving them in amongst each other based on their cooldowns. Trying to use both at once, and finding keybind space for everything is going to be tricky. The upside is if you can pull it off you'll have both a lot of burst DPS, and utility. The downside is that neither of the pets can really tank well enough without proper healing, and trying to focus on healing just one while you have the other class DPSing isn't going to be easy.

I prefer to use my hunter with a priest, and for farming I've yet to find anything that can match them. I just bubble the pet and send it out to round up several mobs, then recall it to right in front of the hunter, where I spam volley, while bubbling/healing both hunter and pet. At 66 I was pulling 6 bog lords at once in zangarmarsh (I lose pulls if I go for any more), and it was a laugh watching all the bots run round trying to find something to kill.