[dungeonguru]

Wow, that's so munchkin it hurts my brain.

I don't think it can be done. A single class that has two separate spellcasting methods can't be done as far as I know.

If you disallowed dual-spellcasters, you could try to make one-offs by copying one of the base classes into a new class and then bootstrapping the missing abilities of the secondary base class into this new class.

The second trick would be to how you want to handle the subclasses. A subclass is considered a custom ability. You could allow two on a base class, but you would have to figure out scripts that choose the timing of granting the subclass choices. Like Clerics choose their domain at 1st level, but Rogues choose their archetype at 3rd level, so your scripts would have to increase your class field values at certain points.

So, a one-off build is possible but trying to make a generic Gestalt might be too much to try and bite off.