You can safely parallel them, though if they have a large gate capacitance you may find that the oscillator has to be redesigned a bit to be able to drive all those gates.
What I'm concerned about is that if that transformer isn't "perfect", you could be getting some nasty high voltage spikes that are blowing up your transistors. If you have access to the parts, try adding something like this. (If that resistor gets hot - not warm, hot, then you have a problem that a little snubber isn't going to solve. And don't touch it to find out, at least while the circuit is running.) It will eat some of those spikes. The parts are important, though - the resistor can't be too low in power rating, the capacitor has to be able to handle a good bit of voltage, and the diode must be a fast rectifier (no 1N400x or anything).
Edit: Er, yeah, I'm aware that the center tap goes to +12, not ground...