what kind of male rat snake do you have? If closely related it is very likely you will get viable eggs. As I am no expert in hybridizations of snakes (or any species for that matter), can't say if they will have viable healthy offspring. Only thing you can do is wait and see.
I also suggest you always house different species of snakes separately, whether they may breed or not. Snakes for the most part are not social animals and unless you completelyunderstand the environmental needs of the animal, having multi-species in teh same cage isn't a good ideal. There is a chance of cross contamination of bacteria, parasites and diseases that can affect both species. Sometimes one species carries something that normally is harmless to it, but could be very dangerous to other species. Though many parastes are very species specific, they may have no problems with closely related species. (ie a flea on a wolf wouldn't have any problems feasting and surviving on the family dog. Both are closely related species of Canine (on a side note, I am assuming the common dog flea is able to live it's life on wolf or dog but it is always possible this particular example isn't a good one LOL)
I would keep both snakes housed separately, then decide if you do get fertile eggs, if you wil incubate them. Hybrids,though sometimes quite stunning in appearance, are not always easy to sell or appreciated. If the combination happens to produce rather plain or drap animals, it would be much harder to find homes for them. Of course you could keep them all for the entirety of their lives, that's up to you but something you will have to consider. If the rat snakes are fully compatable and you produce baby rat/great plains rat, (they may actually be available,I know there are hybrids with the great plains rat out there, so you should be able to find examples of what they look like online).