Two boards connected - 20W

Saving pins by using an ADC channel to read 3 buttons is OK for just one board. But as soon as I connect two of them, the voltage levels are off. This was to be expected, as now two pullups are there, but even after removing one of them... doesn't work. The SMD resistors also vary quite a lot it seems.

As I have another pin on the bus, I might later create some sort of message system to let the boards talk to each other (half duplex).