With todays point of view multiboxing in d3 is not as simple as in wow, but with the change of mf, gf and +xp bonuses are shared across the group multiboxing for farming advantages can be possible.
With every new player in the game, the game gets more difficult.
But what if you are overpowered, and you can kill fast enough without any help, also if there is a 2nd player in the game, who stays all time in town?
The 2nd player in town is your 2nd charakter on the same computer or on a laptop beside you.
If you are doing gf or mf runs, then your 2nd charakter wears full gf or mf gear. No other stats are important this time.
You get 50% of the mf and gf from this charakter and 50% from your own gear. So you can focus more on damage. If you play solo you would get 25% from your follower and 100% of your own gear. The 2nd charakter has more slots then a follower. In comparison you can get more mf and gf with a 2nd charakter staying in town, but your killspeed alone against stronger mobs must be in range of the killspeed with a follower against weaker mobs.
gold find runs
If you are doing this at nightmare or at beginning of hell it could be possible to achive a killspeed to get a better gold / hour ratio.
magic find runs
Every time you meet a unique / boss monster you can teleport the 2nd charakter by klicking on the banner, so you can double the loot from the mob. From this follows that we must not be as fast as alone with a follower, because of the 2nd drop.
In inferno I think it would be impossible, but in hell maybe it's possible.
+xp runs / leveling a new charakter
With a good prepared item set for the leveling charakter, I think it should easily possible to rush trough normal and nightmare easily, while a 2nd charakter with max +xp items staying in town.
If selling charakters will be a profitable business, maybe this is the way to go to level your charakter as fast as possible.
I don't know if this can work or my idea has any error.