Even better you perform partial linking on each library and them make an archive of the two resulting object files. That way it operates like shared libraries would

You do partial linking with 

 gcc -r --nostdlib

so either instead of making the intermediate archive or after 
reextracting it, run

 gcc -r --nostdlib $CFLAGS $OBJECTS_A -o $LIBNAME_A.o
 gcc -r --nostdlib $CFLAGS $OBJECTS_B -o $LIBNAME_B.o

then

 ar -cr $LIBNAME_JOINED.a $LIBNAME_A.o $LIBNAME_B.o