I have a Nikon SB900 which I use exclusively for macro photograhpy.
Would it be a good idea to combine in with a ring light/flash for some extra fill light? Or would it be better to buy another external flash similar to the SB900 and have one on each side of the camera?
It depends on what sort of macro photography you're doing. Macro ring flashes are used as they allow for a very mobile set-up, provide a crisp even light that doesn't get blocked by the lens (like a hot-shoe flash would) no matter how close you get.
However, if you don't need the mobility or you're not shooting handheld then they don't offer any real advantage over an off-camera SB900, unless it's the ringflash look you're after.
You can create some really good light setups with just one off camera light and a few reflectors.
