Interface units, Duos, Duo-Rs, and Duo-RXes all have the exact same 2KB internal capacitor-backed RAM for save memory. Yes, it's tiny and will fill up very quickly; this is why things like Tennokoe Banks and Memory Base 128s are great. But see my post on page one for lots more details about that stuff.
On the note of the MB128, I have one thing to add about that -- it not only holds a charge for a little while (yes, it's short, don't expect that internal backup to hold a save for long), but also can draw power from the system, so it'll also hold a save with no batteries in it as long as you don't turn off your system. Interesting. There is a light on it telling you when the batteries are dead (or low I hope), but if you see the light it might be too late, so maybe replace the batteries every six months or a year or something, if you don't want to lose your files... one set of 4 AAs does seem to last six months at least, maybe more. that it gets power while the system is on is nice, though. And the batteries go in separate holder you remove, so it'd be easy enough to take out while power is on I'd think, as long as you're a little careful. Though with that internal capacitor saving things, that shouldn't be necessary.
Also, I have my MB128 plugged in to my (region-switched) TG16 + base unit + Turbo CD drive, then with a controller adapter connecting the MB128 to the system, and tested that with a Japanese multitap with several controllers plugged into it in a game. Yes, it all worked fine.