I have been working on replacing a broken proprietary stepper driver with an arduino and some L298N dual H-bridge boards (available for $5 or less on ebay) and they do work when used individually.
However, when I wired two in parallel so I could deliver more current the setup just sorta vibrated like it didn't have enough voltage, and got very hot when driven at 12v. I eventually tried 35 and fried both.

So here's the question why do they get tweaked out when paired? The L298N boards are very dumb hardly more than the dual H-bridge with diodes, if triggered together I thought they would act as parallel transistors and share current load but this is obviously not the case. Any ideas?