Thanks for the bug report.
Numpad's enter works normally on Linux, but I just tested the web build and it didn't work there, so there's indeed a bug in that version (and I guess that's what you played)... I'll look into it later.
You can select "mode B" to control a character with the arrow keys, but this mode has the actions mapped to X, C and V, so it's not ideal for controlling both characters by yourself. Setting a characters to "mode A" (WASD + space + left shift) and the other to "mode D" (UHJK + L + G) may be a better option, since NumEnter isn't working...
If you have a dual stick gamepad, I highly recommend using the gamepad's "mode C" for a characters and "mode D" for the other. This maps all actions of a character to the left hand (left stick + left shoulder buttons) and the other character to the right hand (right stick + right shoulder buttons).