You can extract from the old libraries those object files you wish to incorporate in the new. But there really isn't much point in worrying about it; the linker will only link those object files that are necessary, unlike a shared library where all the symbols defined in the shared library are available to the executable (not that it uses them all, usually).
The old-fashioned way to do the job would be:
mkdir new
cd new
ar x ../old1.a
ar x ../old2.a
ar rv ../new.a *.o
cd ..
rm -fr new
After the two x
operations, you can weed and whittle the object files to keep what you want for use in new.a
.