You could get a Raynox DCR-250 closeup filter which would make your two lenses I think both macros. Or you could go a reversing ring if the filter size suits. I have a thread where I compared all the different variants.
90mm macro
50mm and reversing thread
90mm and Raynox DCR-250
90mm & 50mm reversed
50mm & Raynox DCR-250.
I think thats all I covered. The reversing ring is cheap but means manual focus and manual set up. But we can help there, the quality is slightly less, but it gives you a feel for extreme macro. The DCR-250 is $250 AUD (I think) and can be added to a 42mm to 67mm filter size (I think). This improves your close focal distance by 2.5x apparently.