Yes, Nintendo made a special kit with a box that modifies one side of the red tent into a Playchoice, the box gets attached underneath and hold 2 boards
I have tested Dr. Mario in a single counter top playchoice, a Dual Playchoice and a single Red Tent playchoice and all work, I am not sure what gives on your cart, if you want you can ship it to me and I can test it to see if its a cart or pcb issue
I did my first cap kit about a month ago on my DK jr. Monitor, it was a sanyo ez20, I have to tell you it was easier then I figured, also it's fun to do a cap kit! Give it a try you will see