Not sure what you mean exactly, but if you want to:
a. open a mail merge main document with data source A
b. merge to labels
c. change the data source to B
d. merge to labels

Yes, you can do that.

What you cannot do is attach both sources A and B and do the whole thing in
one go. At least, not without using code, and even then you may not be able
to do it.

Peter Jamieson