You still keep the other 2 mechs, so you can switch between the two. Unfortunately, only Joshua with Rim as a subpilot can pilot Forte Gigas. I tend to switch between the two, mostly so I don't have a squad be a lone. Just remember that if you switch from Forte Gigas back to the other mechs, that you also have to reassign Rim to her original mech or she won't pilot anything.
"I've just remembered something...He was not as naive as you are. That's what made him a hero." Zero from RMZ