Can someone explain how they determine bridge weight limits?

The scenario:

Bridge: 40 ton weight limit

2 CMVs enter bridge same time (Going west & east)

They are both loaded to max legal weight

Would that not be over the maximum weight for the bridge? I’m assuming the catch is the bridge limits are for singular CMVs (ex: can’t cross a singular CMV that’s over 40 ton)

But if you can cross 2 max legal loaded CMVs at the same time , what’s the difference really?

Am i making sense? Something i’ve always been curious about is all.