Increasing 12V Rail current on power supply for mobile computing
You question is about the viability of your two suggested options.  Both options are essentially the same thing: load sharing between two power supplies-- in one case it's two of the same supply model, and in the second case it's two different supply models.

Load sharing between two switching converters requires some shared loop control to operate properly.  Some power supplies have connections that facilitate ganging of supplies like this.  It looks like the M4-ATX does not.

If you try to just tie them together, you will have two supplies each with different internal control loops that respond differently (that is, with different timing) to the downstream load transient conditions.  This can result in poor regulation and/or oscillation of the shared 12V output.  This could be minor (you don't know it's happening), major (what's that sound?), or catastrophic (what's that smell?).

Your best bet is to get a supply that can support the full load.