I'm still at this. My alts are 67 except for the hunter who I've decided to level ahead of the others. She's 73.
The speed of levelling is atrocious. There are no shortcuts. If you just play as your tank and run the alt through instances, it's possible to get 20-25% of a level but it takes you longer and longer to solo those instances. Some bosses are damn near impossible to solo, too (like Keleseth in UK is a cheating frost-tomber)
The other option is to use the 80 and have the 65s (I say 65 because that's the earliest you'll be boosting in Northrend, but UK is still good for XP up until 74) assist. They're not useful at all at first, just sporadic lucky hits. As time goes by they get better and better but by that time it's not really worth dragging them around in the same place because the XP has slowed so much.
Today I took my 72 to Halls of Stone (that's the minimum level to get in). I had to ask a warlock friend to do the summoning, because I gave up on my warlock a few days ago and don't have an 80 on that account to pass Cold Weather Flying down.
I used 4 80s and the 72. I had heard that being within 9 levels means no XP loss, but also that if the enemies are green or yellow to the 80 then you get significantly more XP.
I was getting 1300 per kill or so. That's cool and all, but it meant that a full clear only got me 6% of a level on the 72.
So I'm back to questing. I hate it, because I feel like a tool with no free will (everything's so linear) but it really is just as fast or faster than running them through the highest dungeon they can get into.
At 75 I'll change my plan. That's when the highest dungeons open up (HoL, CoS, UP, ToC). I think you can park an alt at the entrance area in UP and still be in range to get XP from the whole place - but I'll have to test that later.

-Gun Drak was pretty good this way. I'd kill the snake boss, then jump the lowbie down and park them on his platform. They'd be in range for master loot and XP for the entire dungeon (at least, taking the swimming shortcut from colossus to Moorabi).

The rest?

-Utgarde Keep: Great at first. Watch out for Random Second Party Targetting (RSPT) mobs like Dragonflayer Strategist, or the first two boss fights. If you have a clothie you're probably better off leaving them around the corner.
Keleseth will ruin all of your plans by frost tombing your tank, summoning skeletons, and running away. Enjoy your frustrating and agonizing death.
-Violet Hold lets you park them outside of the locking door with the human NPC, and still mooch. For the most part, they're safe, except some bosses zone-wide AoE through that door. The void does, not sure which others do. Many of the portal guardians give XP for being special; this is often double a normal enemy's amount. You *might* be able to solo it, at least some of the bosses, which would be a pretty fat XP boost.
-Azjol-Nerub is hard but fast. You probably can't solo the first boss fight on any one character, though, so like the others you have to bring 2 or more, preferably a team. The best part of this instance is the two quests you get in the early 70s; since it's such a short instance they're quick XP, and are along the way anyway.
-Old Kingdom has great "Mob density" they say, but park your alt around the corner all the time no matter what. If those shadowcasters start Shadow Sickle from far away, or the web winders grab them, or a million other possibilities of what can go wrong do, then you end up wasting a lot of time with the boostee dead.
-Halls of stone: Skip Brann, skip the end. Just reset and farm the first two. If your alt is closer to 72, don't even bring them down that tunnel toward Krystallus; they can mooch XP just fine from the main hallways. If you bring them down that hallway you might hear "CRUSH...." as soon as you take two steps and next thing you know, here comes Krystallus and an army of Iron Dwarves & Shards. Good for a laugh.

Before I came to Northrend, when the alts were 63ish, I tried a little experiment since heroic keys for Coilfang Reservoir and Hellfire Citadel can be bought with Honored reputation. I ran them through Heroics. Conclusion: Not worth it at all.
They'd get the same XP for a +2 or +3 as they would for a +10.