Pretty much... if you have 1x an identical toon, you have 1x the dps.
With 2x the toon, you have 2x the dps and kill twice as much in the same time period.
Again 3x the toons, 3x the dps, kill three times as many in the same time period.

With 1x you don't get any group bonus.
With 2x its a small bonus.
With 5x its a larger bonus; not huge but if you're not running out of mobs it will level you faster while grinding.

With 1x you hardly ever run out of mobs, and collection quests are the least pain to do.
With 5x mobs, your efficiency hurts you when you have to wait for respawns and collection quests truely suck.

It's a balance between the number of mobs you have in an area vs your kill speed.
5's can do instances, while a tandem of two probably not. That's another consideration.