they actually made those at one point. it's called a Reverse Duo Tap and yuo can plug five duo sized controllers into it and then plug them all into your TG16 (they also made a Reverse Turbo Tap for the duo). I got one years ago when i ordered a duo tap from a store by phone and they accidently sent me a reverse tap because they didn't know the difference. i was confused and thought i had a misfit duo system or something until the local TG16 guru was like "oh yeah, that's a reverse duo tap." i later saw some other online seller with them listed on a website (this was in 98 or something)..
I really don't want to part with mine, but i've heard you can buy a Mac cable that works fine for converting the different pin sizes.