I've setup my headlight wiring just like the diagram calls for on my 1950 F4. My question is should I have voltage on both high and the low beams when the high beams are on. If so would a bad dimmer switch cause only one circuit to be on at a time?

 

It's either wire is hot, not both. Separate filaments for high and low. The dimmer switch selects which one. NAPA sells an exact replacement dimmer switch, NAPA AUTO PARTS Dimmer Switch

Real common for them to not connect on one or the other, to hang up in between, etc. People aren't gentle with them!