By having parallel devices that are capable of carrying the full load, you're increasing your MTBF of that particular section of the circuit. Then you'll need to look at the MTBF's of the remainder of your circuit to see what it's weak points are - like perhaps the Zener diode, or the gate drivers, or perhaps the width of the board traces. You can spend a lot of time re-engineering things to be "bulletproof".
Having two power MOSFETS in parallel on one hand is convenient, because if one starts getting warm, it's internal resistance increases, and the other MOSFET will carry the load until the hot one cools down.
OTOH, you double up on your gate capacitance and output capacitance. The former requires more robust drivers to keep the gates supplied, minimizing transition time and resultant heating. Increased output capacitance will lead to larger snubber capacitors being required. It will also increase your costs of both the BOM and assembly time.
Oh, for your negative supply - have a look at Linear Technology's LT1054. It's a switched-capacitor voltage converter that can invert your positive supply (up to 15v) at up to 100mA.
Having two power MOSFETS in parallel on one hand is convenient, because if one starts getting warm, it's internal resistance increases, and the other MOSFET will carry the load until the hot one cools down.
OTOH, you double up on your gate capacitance and output capacitance. The former requires more robust drivers to keep the gates supplied, minimizing transition time and resultant heating. Increased output capacitance will lead to larger snubber capacitors being required. It will also increase your costs of both the BOM and assembly time.
Oh, for your negative supply - have a look at Linear Technology's LT1054. It's a switched-capacitor voltage converter that can invert your positive supply (up to 15v) at up to 100mA.