Weird setup

You can get a banjo fitting with a -6 male spout on it and 16mm hole. Cant find a part number for it, but I have one on my PAS pump.

I'm sure it will flow far more than you'd ever need, and as it will likely be the return anyway, it's of less importance.

A single -6 would support well over 600hp...so using it on one side will hardly be pushing things. The hole in an M16 banjo will be at least equal to that of a single -6.