Odds are your savannah monitor would see the rat snake as a fun prey item, I wouldn't reccommend it.

Examples of snake-lizard pairs that work well are things like:

Rough green snakes with anoles, house geckos, and small skinks.

Er...actually, that's the only grouping that I have seen succeed. That doesn't mean there aren't other pairings that would work, but I haven't seen them. I did some quick research, and it has so far confirmed my knowledge...that pairing species of lizards and frogs is much easier than pairing snakes with other types of animals.