Multiple voltage sources
A potentiometer is just two resistors with a central tap, where you can vary the percentage of each.

Your first scenario looks like the left schematic below.
At 50% on each there is 2.5V on the output.

With one at 5V circuit 2 is your setup. Here you have 5V on the output. R7 is shorted out. Same sort of thing would happen if one was at the ground end. It would take priority.

Anywhere in between on both and you need to do the resistor divider math. 







simulate this circuit &ndash; Schematic created using CircuitLab

If you mean some other connection setup, you need to add that to your question as a schematic.

BTW Connecting pots in parallel to the rails like that is a bad idea. If one is at one Vcc end and the other at ground, you will short out the supply. One at an end and the other close to the other will fry the latter pot.