@UKHeliBob Thank you; I'd gone through that doc. My pin numbering in the IDE is set to "By arduino pin (default)". I'm not using any external libraries in this test script, so I don't think I need to use the GPIO numbering scheme. Let me know what you think, thanks!
@xfpd Thank you for the suggestion; I actually had them without the D by default and tried the D just in case. Can you explain what you mean by having the motors backward? As I mentioned, everything worked fine with an Uno, and the pinout on the driver side is the same now as it was then. It looks like you're saying I have the EN and the IN1 pins switched, but if you look at the schematic for the driver, the order is V_IN, EN, IN1, IN2, GND. If you're just saying that the motors aren't all going the same direction based on the write state, that's not my concern at all. Let me know, thanks!
The main thing I can't wrap my head around is how this single Arduino pin can affect the operation of these two different motors.