I have had a lot of success with a 2-man.
When I went to 3 licenses, I just ran 2 for a long time until they were both geared (this was a mistake, would have been faster to gear all accounts at once).
The options are a little more limited with 2 accounts than 4, so I would run 2 DPS characters (no ZDPS), but you need to keep in mind survive-ability when you do this.
In a 3 man you can add a ZDPS character to keep everyone alive then do 2 DPS focused characters. This is where it really starts to be apparent that you're smashing through GRifts way past your gearing level.
To explain the image a little better, it's technically comparing 2.3 to 2.4 in terms of monster damage.
The entire chart is based on monster health in 2.3 (Blue Bars):
1 player: Monsters at 100% normal monster health.
2 player: Monsters at 150% normal
3 Player: Monsters at 200%
4 player: Monsters at 250%
Let's say that there is a monster that had 10,000 health during patch 2.3:
[TABLE="width: 500, align: center"]
[TR]
[TD="align: center"][/TD]
[TD="align: center"]
HP in 2.3[/TD]
[TD="align: center"]
HP in 2.4[/TD]
[/TR]
[TR]
[TD="align: center"]1 player[/TD]
[TD="align: center"]10,000[/TD]
[TD="align: center"]~7,000[/TD]
[/TR]
[TR]
[TD="align: center"]2 player[/TD]
[TD="align: center"]15,000[/TD]
[TD="align: center"]~12,000[/TD]
[/TR]
[TR]
[TD="align: center"]3 player[/TD]
[TD="align: center"]20,000[/TD]
[TD="align: center"]~18,000[/TD]
[/TR]
[TR]
[TD="align: center"]4 player[/TD]
[TD="align: center"]25,000[/TD]
[TD="align: center"]~25,000[/TD]
[/TR]
[/TABLE]
Conclusion: they have scaled the monster HP to be spread more evenly in 2.4.
Yes, that mean's there is less advantage to having more bots when compared to solo monster health.
What it doesn't account for is the build synergies, crowd control, buffs, de-buffs.