Mazzilli flyback driver mosfets keep failing
Short version: Build a Mazzilli flyback driver, not the schematic shown above.

I recommend you have a look at Mazzilli flyback drivers online and compare your circuitry to theirs, as your circuits connectivity around the MOSFETs differs to Vladmiro Mazilli's circuit and I think that is the source of your fried silicon.

I would expect to see:

1) a resistor divider to each MOSFET gate from the 100VDC rail (from 100VDC to 100VDC RETURN, with the centre of the divider at the MOSFET gate).  This means two resistor dividers.  

2) a 12V zener (or thereabouts) between each MOSFETs gate and the 100VDC RETURN (i.e. the bottom of your 890uF cap)

3) a schottky diode (anode) from each MOSFETs gate pin to the other MOSFETs drain pin.

The smaller value resistor (my guess 10k 1W or 5k1 2W) is to turn the gate on quickly but not too quickly.  See the MOSFET datasheet.  You will have to calculate the value and power dissipation of this resistor.  The zener can be circa 12V 1W (in the above examples it dissipates 0.1 or 0.2W)  The higher value resistor (my guess 100k) just needs to be enough to discharge the FET when the circuit is no longer powered.  Under normal operation the schottky diodes discharge the MOSFET gate when the other MOSFET is switched on.

I hope this helps.