Good point, and don't forget the WM T-34. Actually, the Skyradier Mach-1 can serve as both a trainer and a "second" plane because of it's semi-symetrical airfoil and shallow dihedral angle.

Another option is fron nitroplanes.com. I never thought I would say this about them but this thing flys great:

http://www.nitroplanes.com/nilosuaetr40.html

It looks like a Tiger-2 but has a flat bottom rather than a symetricl airfoil. Sure the manual may be pretty bad and I've heard horror stories about customer service but the plane is priced right and seeing it fly is impressive.

The GP Cherokee is a good choice but I think the dual aileron servos and flaps hurt it as far as second planes go. I think a guy should be able to pull the gear right out of the trainer and drop in the next plane. The Cherokee requires the addition of at least one extra servo or maybe three is flaps are used. Plus the flaps could just add confusion. A Pitts muffler is also required. It looks and works great but adds to the cost. It flys excellent though.