It is possible to combine two signatures in a .sig file. But the representation of the results is not good if one or both signatures are invalid.
If the first signature in a file is invalid, only the result for that one is displayed:
If the first one is valid, both results are shown, but you have to look closely to see which signature was invalid:
Ideally, one would always be presented with both results and see at a glance which signature is valid and which not.