Sawbwa and rummy nose wouldn't work because their water needs are so radically different. They might drink together a little, but.
My favourite version of this is Lucania goodei and Heterandria formosa. The first is the Florida Blue Fin Killie, and the second is the 'least killifish', which isn't a killifish but a tiny livebearer. I caught them together in Florida, and they don't look alike. But I was having trouble raising the blue fin fry, and realized they looked almost identical in their markings to the livebearer fry in the next tank. I put the two species together, and bingo. The fry shoaled and I had plenty of both.
All of a sudden where I'd been getting 2 or 3 bluefin fry a month, I was getting 10 or 15 in with the Heterandria.
There are lots of mimics - the Cory tetra thing
@Innesfan mentions was a cool thing to watch when I got both.
It doesn't work with territorial fish, where they seem to compete. But with shoalers, it can be interesting.
I put a Tateurndina ocellicauda peacock gudgeon in with a male Fundulopanchax nigerianus killie. They come from a world apart, but have remarkably similar colouration and size. The killie just looked at the gudgeon with a "nice try, wannabe" look, and they ignored each other.
