This IS a problem, most of (if not all) multi-boxing guides/videos are focused on shaman based or single class groups. The guides/videos that are not shaman based or single class are non-multiboxer and thus do not take into account positioning or other special circumstances needed for multi-boxing.

That being said, here is what I have found so far. While I have not run all of the instances, and I got quite frustrated in the 71-75 range, I did get a few bosses down.
I run a Warrior tank, shadow priest, balance druid, fire mage, holy paladin on three computers and FTL.

1st boss in UK did take a few attempts to figure it out, but the key is to keep your slaves on follow and keep them on your tank, add a line to your tanks spam macro that says /target IceBlock (or whatever it is) and just burn him down, when the skeletons spawn hit your aoe on your tank and ignore them. They have low HP and will die but will just respawn as soon as you kill them so no use worrying about them.

The second boss in UK I do not follow the standard guides. They say treat it like romeo and juliet but I just tank the melee guy and have my casters burn down the caster then turn on the melee, the caster will be loose and shooting everyone, but I have not been able to keep agro on him with my tank when he is alive, so I see no difference. Just make sure the caster dies first.

Ingvar: I have some thoughts but I never beat him at level, and have not bothered to go back and try again.

The next Instance I tried was Drak'Tharon Keep
Started working into the isntance but the first couple pulls caused me some problems, sheep the humanoids and burn down the little guys (they drop a black void on the ground that does decent damage). Trollgore was a push over tank and spank. I even had my pally tossing some spells on him. have the tank on the stairs and your group at the bottom, the explosion is horizontal so your group should be below the damage area. That is unfortunately all the farther I got as I was unable to work out a strat on the summoner before I got frustrated.

Then we have Gundrak, the trash is straight forward and pretty easy to control, there are only one or two pats you need to keep an eye out for. I would clear the trash here for exp then reset. The only boss I was able to down was Moorabi, simple tank and spank, try and interupt his changes but no biggie once you miss it. Slad'ran I had trouble with, never quite found the right position so the aoe would not tear me up and the Drakkari Collosus just aoe'd me down in his elemental form. Everyone says it is cake with a nature resist totem, but I do not have one.

The last one I did was Halls of Stone. I would clear all the trash in here for exp and is how I finished off to 80.
Krystallus is a joke once you get your positioning down. There are 4 rock outcrops on opposite of where he stands, a big one on the left as you are facing him, place a ranged dps here, a smaller one to the right (healer goes here), a single rock that you skip, then another outcropping you place another ranged dps on. Put the last ranged dps up the wall abit so you are still out of range. With this, once you get the exact right placing, the knock back will not move your teams at all and they will all be within range of healing and out of range of the other shatters. I believe I have an advantage on this one with a prot warrior though, as soon as he does the knock back I just use charge and I am right on top of him again so he never moves.
Then it is just a tank and spank burn the bastage down.

Maiden of Grief: This one I did just like the normal group strat says. Park the ranged in the center and circle them with your tank walking backwards. She does an agro whipe with her Shock of Sorrow, taunt how ever makes it through this, so I just taunted right before the cast completed, I would be on top of the agro list when she was done and would break my tank free. If this does not work for your set up, just step into one of the black voids right before the cast is done. You have to make sure your tank breaks free or you are toast. It also helped with my priest and mage mostly using channeled spells for dps as this causes them to turn with the mob and can keep up dps where the druid can only do it for 50% of the circle.

The event I was never able to beat and thus never made it to the last boss.

The rest of the instances I have not actually done as a group though I have run most of them on solo characters. I am just pushing to gear my tank for raiding then will jump back on other stuff. I hope this helps at least a little bit.