could a humbucker be made from 2 P90s?
Sort of... If you have 2 P90's and one is RWRP then you could wire the two pickups together and it would buck hum but it's not going to sound like a humbucker because the bobbin is a different shap. To wire the pickups like a humbucker simpliy wire the hot of one pickup to the hot lug on your switch or pot and then wire the ground of that pickup to the hot of the other pickup. Then solder the ground of the second pickup to the guitar ground.
If you want a humbucking P-90, DiMarzio makes one, and I believe Seymour Duncan has the P-Rails, which is a P-90 and a rail SC in humbucker form that can be split for either of the SC voicings..
i have the DiMarzio one. imagine the fattest sounding guitar you've ever heard, well its a bit fatter sounding than that. very warm also, not a bad sound for blues or jazz or something mellow