I felt this same problem with IRFZ44N. I tried hard to solve the problem, finally I changed the zener diode 12V replacing by 6.2V. as the gate voltage is no required too high, so it was sufficient. It was on my design. you can try this too. and also change the MOSFET. replace with more VA(low voltage high current) as IRF540. You didn't write anything about the VA of the UPS. If the capacity is more than the MOSFET can drive then it will burn out easily. and a delay should be kept between inverter mode to main mode. or a high current flows after the inverter mode changes to main mode. this is a serious problem. if there is no time delay for charging between the inverter mode and main mode then its the main problem for your UPS. in this case you the MOSFET capacity should be at least 2times than the inverter capacity. if you cant add more FETs then find out the gate voltage of the both channels. if the gate voltage are not same then the FETs are burning in inverter mode, and if its ok then FETs are burning at changeover mode+charging mode. in this case, add a diode(6A) with each fet (S to D). this will help the internal diode to take more current. any way another case may happen, check the ups carefully. is the burning process is happening just at the changing time??? or within some ms??? then the problem is the oscillator is not beibg stopped while the UPS goes main mode. if so then the MOSFET have to carry a huge current, charging current + back up current. if this happen, then nothing can help you without changing the UPS.
I think all these will help you.
Thanks
Mithun