Let's say A = (MWI is correct) and B = (Copenhagen)

The equivalent of "A OR B" is the statement "either Copenhagen or MWI is correct", and I'm sure everyone here assigns "A OR B" a higher prior than either A or B separately.

But that's not really a theory, it's a disjunction between two different theories, so ofcourse we want to understand which of the two is actually the correct one. Not sure what your objection is here.

EDITED to correct a wrong term.