Can't play two player games with a dual arcade joystick

Hi, everyone.

Retrobat recognizes both my joysticks on my dual arcade joystick but I can't play as a second player on two player Mame games like Bubble Bubble, Mario Bros, or 1942.

If I plug in each joystick separately, they can be used to play one player games. But if I plug them both in at the same time, the second controller doesn't work i.e. I can't add coins to the Mame games using the second controller and I can't join in as a second player.

Retrobat does see both controllers at once because when I plug them in, retrobat indicates that it detects two controllers and lists both of them in the "map controllers" screen. Although it doesn't allow me to configure the second joystick if both are plugged in.

Is there a procedure or guide to follow so that I can play two player games with a second controller / joystick?

Thank you.

TH