I do want them to have a common ground. I'm just a bit confused is all.
If my two circuits have common ground, and I have a 10K pulldown somewhere in my 12V circuit and a 10K pulldown somewhere in my 5V circuit, then somewhere in my circuit this is happening:
+5V --10K -- GND -- 10K -- +12V
Which is not dissimilar to the circuit with the switch. The only difference is in the switch circuit I can break the connection to ground while leaving the 10K resistors connected. So assuming the setup above with two pulldowns is okay, is being able to break the connection to ground what makes the setup with the switch bad? (Without diodes in place.)