This is not possibly - today, at least.
Using multiple /DEF options to POLIB will combine all symbols seen, but only the last LIBRARY name will be used to build the import library; all symbols will be associated with this (DLL) LIBRARY name. The next release will warn about conflicting LIBRARY names from multiple module-definition files, but otherwise the current behaviour will remain.
Combining multiple import libraries (LIB files) also fails because of the non-unique symbol '__NULL_IMPORT_DESCRIPTOR' used by Microsoft (the failure comes at link time). This could possibly be fixed by special code, but since I don't think combining import libraries is very useful/important/interesting, I will not waste much time on this. If it's really easy to fix, I will add something in the next release, otherwise I will not bother...