Multiboxing DH hits a wall around 37 or so. Damage is not a problem, but survivability is. EVERYTHING 1 shots u, even white mobs. Since u cant have a follower with Unity to duplicate your toughness, being a glass canon turns into a big problem. If u get a bad rift, which is extremely common thanks to Blizzards bad RNG/balancing, u will get Act5 mobs. They are fast, they jump, charge and dont give much rift progress regardless of their difficulty. U will prolly die several times and fail the rift. Anarchs and Winged Assassins are a huge pain in the butt. Even with all the cold damage they get, they are still pretty fast and can charge or jump u, sometimes before u even see them. It only takes 1 hit and 1 or more DHs go down. Awareness is a must, but even with that, u will die. Chanon Bolter wont save u here cause in higher GR mobs take quite a while to kill and the taunt will run off before u kill them.
That's just white mobs. Elites are a different story. Jailer is retarded and is unavoidable. Unless u have 4 Julia Cameo amulets with good rolls, then u will insta die to these packs. Solo is not that hard to deal with, cause it's all about position and range, but multiboxing is something else. Then there's is cold too which also will instakill u in less than a second of standing on top of one of the ice thingys. And to enjoy it even more, u have Thunderstorm.
Basically u cant chill or relax for a second. Anything and everything kills u. GR35 is not difficult so u can upgrade all your gems to 25 with a good DH group, but beyond that its a little bit difficult. Wizards are a little better IMO because they have far more toughness and can die 2 times (passive + firebird's 4set bonus). I use a 3x Wizards and a support WD and things go pretty good. Still need to work on the wizards gear tho. My main is very good geared and im at solo GR38, can prolly do 40 without problems but need to get the right rift ... RNG!
Non multiboxing parties are always 2 pure DPS DH, 1 zDPS monk and 1 support/zDPS WD. Basicall, it's just the DHs doing damage and the monk and WD providing buffs/debuffs to multiply the DHs damage. WD will use Tiklandian to root all mobs and disable all elite affixes, spam Big Bad Voodoo, Mass Confusion and Piranhas (30% + 20% + 15%). Monk will stun also, pull everything and debuff with Mantra of Conviction, Inner Sanctuary and Crippling Wave.
DH is a lot of fun and its a very different playstyle but can be frustrating. For T6, I suggest u get a Boon of the Hoarder Gem, Avarice Band and a Goldwrap. That way u can forget about survivability. For higher GR, Wizards might be a better choice for multiboxing.
PS: I'll try to make a video during the weekend.