Just started out, learning the ropes slowly but it's quite amazing what you can do once you put some time into setups.
Having a main account before starting multiboxing helped quite a bit with leveling up.

Hardest part before you can really boost all your characters to 70 fast is getting each account into adventure mode, I'm not sure what the exact mechanics are but once you kill malthael you unlock adventure mode (60+?). After that it's easy as pie to level up characters.

This only works if you have a booster main account: 1 main + 3 alts
Load up a T6 adventure mode game with one of the alts.
Select a4 bounty hellrift, join with all characters.
Leave low level character at waypoint, proceed with main character. They'll get credit for everything just standing there.
Clear bounty + very often spawned cursed chest.

1-50 you'll oneshot pretty much anything
50-62 can will get slower, depending on your main's gear
62-70 cursed chests will become very hard to do, trick here is to create 4 seperate games and fish for "clear hellrift 2", yielding significantly more xp and can be cleared very easy.

Bottleneck is mostly 65+ depending on your main characters gear, but in hindsight you might as well just level up the other 3 multiboxed in adventure mode rifts from 65-70. Either route should take about 1-2 hours from 1-70. Anyway was my way of doing it, I'm not aware of faster ways currently but happy to learn if there are.

Current teams and experimenting:

4x WD's : ~300k dps farming normal bounties currently, rifts on t1 took about 15-20 mins to clear. Goal here is to gear up and push 600k dps while farming a1 bounties for ring of grandeur. Skillwise I'm not really set yet on what is best since the damage isn't yet that high, switching alot between acid rain/firebat-sycopaths/locust.
4x Wiz: Very tempted to try them out reading posts on this forum, round robin with things like black hole/frost nova sounds amazing. Got amazing luck with 3/4 characters having a firecoat, once last one has one too I'll be giving it a go.
4x DH: Not sure what to do with these guys, firecoats and reaper wraps might make em viable since hatred generation is in a terrible spot right now.

Loving it sofar.