Hmmmm it can be possible that Rust does not support SLI Yet?
Or you can simply try selling your Two GTX 250's and invest on a 600 Series for Nvidia or a R9 2xx Series or 7000 Series from AMD.
OR
The game simply isn't built properly yet and there could be memory leaks or simply not compatible with your machine in a way.
Edit: Hope I helped Bud.