I've been playing around with the controls on this game. I am just trying to make it work with 2 keyboards. I found a way to eliminate the weird issues. It's not perfect since MAME does seem to have a limit on how many commands you can have per input. Basically, using the "not" command I have set it so the hatch will open if only one of the hatch buttons is pressed. If two are pressed, then nothing opens.
For every hatch pad, I have mapped one key from the numpad as well as seven other keys mapped as "not" commands. In order for this to work perfectly, MAME would need me to map eight "not" buttons. Worst case scenario, you have two buttons pressed and one of those hatches opens instead of no hatches.
The weird flash across the screen doesn't happen and, more importantly, no random hatches will appear on your opponents playfield.
Attached is my CFG file. I am using the default keyboard as well as "keyboard 4". If you want to try this, you'll have to figure out what number keyboard(s) MAME is seeing on your computer and make the appropriate changes. Replace "KEYCODE_4_" with "KEYCODE_2_" if your second keyboard shows up as Keyboard 2. Don't forget to enable multiple keyboards.
View my gameplay videos:
Light Gun Games with Aimtrak
Trackball Games with X-Arcade Trackball
Star Wars Episode 1 Racer with Dual Throttle Controls