A question about WeldConstraints?

I'm having serious problems with WeldConstraints. From what I understand, a part can't be used by multiple welds. Meaning if you had a big part called C, and two smaller parts I want to weld to C called A and B, I can't have a weld inside of A welding A to C, and a weld inside of B, welding B to C.

I tried other things, like welding A to C, then welding B to A, which is logical, but also doesn't work. How to weld multiple parts?