Paralleling Mosfets: Can I use a common gate resistor, or do I have to use a separate one for each mosfet?
Depends,
And that depends is based upon your REAL circuit not your intended circuit







simulate this circuit &ndash; Schematic created using CircuitLab

Your practical placement will create something like this (there will be a few other stray inductances but for now this will do).

If you think about the current flow when you charge/discharge the gates it will be


MOSFET driver
Gate resistor
Split path to the MOSFET
via each MOSFET source
recombine at the common reference
via some path back to the MOSFET driver


This loop is one you need to keep BALANCED &amp; ideally minimised. Imagine if due to poor layout/tracking/wiring the right FET's source had 10x the inductance on the gate and/or source, it will switch slower which mean the left FET will experience more of the transient responses. 

In large power devices they use a small individual gate resistor per die &amp; then parallel all the devices up, but they keep the layout really-really tight &amp; equally they are in control of the MOSEFET/IGBT batch characteristics for very closely matched devices . If you cannot do this then it is better to have a separate gate resistor.



Parallel IGBT die on a common substrate

The benefits of a separate gate resistor is, if you need to tune the response of one leg based upon other observations, you can