The setting menu says using NumEnter should use item, but it pauses the game instead.
You can control one character with WASD, but arrows don't control the second one, which is confusing.
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).
Best regards!