1st off I don't believe it's possible to be faster to duo up 2 characters then boost a 3rd, than it will be to simply level the 3rd right off the bat. Even with RAF and granting levels. So if your goal is to dual-box and raid with your friend endgame ASAP then you should just level the characters you want to raid with. Maybe boost some others later if you want to play other classes or increase the # of accounts you're running.
If you're on RAF and want to maximize it then it's probably best to level 1-60 a few classes that you are sure you want to play.

Questing isn't too bad. I leveled a Warlock solo without any help on a brand new PvE server, from 1-80 in just over 7 days played including leveling tailoring and enchanting. You'll probably be slower, leveling 2 toons at once. Plus I've done the leveling grind many times before (1-70 at least 8 times and 70-80 4 times) so I know exactly what to do and where to go without wasting time. But if you follow a decent quest guide mod (TourGuide, Carbonite, etc) it's still going to be pretty fast, 9-10 days as a guess? for 1-80.

2-boxing and questing with a guide it really won't matter much which classes you choose. Just spec 2x DPS and melt face. Maybe you can get your friends to boost you through some of the low levels. Maybe something like level to 20 via quests, boost to 40 in Scarlet Monastery. Then quest to 45 and boost to 58 in Stratholme. Gear yourself with BoE greens or boss drops from Strat, get a few crafted pieces maybe and you'll be ready for questing through Outlands. The boosting won't take but a few nights, it depends on how much you play and how efficient you are of course, but it's pretty fast even without RAF. You can probably cut at least 25% or more off your leveling time with some efficient boosting.