Oscillations with paralleled MOSFET
Power MOSFETs have relatively large Gate-Drain capacitances. 

When they are paralleled, you have a closed AC loop from common Gate to Drain1 to Drain2 back to common Gate. 

What do you need for oscillation? 1) An energy source [check] 2) feedback [check]

So it’s good to insert a resistor for each gate for damping purposes.