Brake Lights

A neighbor today told me my DS brake light was out. I checked and sure enough the top light lit on the PS and not the DS. I took the assembly apart and realized that both the top and bottom bulbs (there are 4) had bulb type 3057 (dual filament). So, I assumed that both the top and bottom brake bulbs were burned out. I replaced both the top and the bottom bulbs on the DS, top works fine now, but still not the bottom. Do I have a problem here, or is just the top one supposed to light with the brakes? If I am ok, why a dual filament on the bottom as wemll as the top?

Does the Silhouette/Venture have two brake lights per side? The actual assembly has 3057 imprinted right by the socket.

This is a 2003 by the way

Just curious...

SN95GT50

The top dual filament are one for night and the other for brake. The bottom is for night light only, not for brake, the other filament has no connection although they use the same bulb.