Marginal increase in power availability. How will you balance the two amps?
The amps have to have the same gain.
Passive biamping gives more power obviously. This would make a difference if you listen to music with a large dynamic range, classical for instance, or if your current single amp is rather underpowered. Personally, I don't think passive biamping is cost effective as an upgrade unless it is just an intermediary step on the way to active biamping. Active biamping is a big upgrade.
Regarding passive biamping, one good amp is better than two not so good amps IMO.