Curious about the way the headlights function

[gavinmosher]

I have a 84 chevy c10, last night my little bro and i swapped the core support over to the 90 suburban core suppt. now when i turn the dims on one bulb lights up in the center, and then when i hit the brights the dim shuts off and the outer turns on....

my main question is, When your bright lights are on are both suppose to be lit? or just the one.