It sounds like you've already made up your mind, I think you should stick with it and report back with problems, successes and pictures. I think I have heard of a lot of big time rainbow boa breeders keeping two females in a decent sized cage throughout most of the year. I do this with a lot of my big girls and have never run into a problem. I don't think keeping two different species, especially two that fill different niches, would cause any more stress or problems than two of the same species together. Of course there is more risk for failure but if you're a competent keeper you shouldn't have to worry too much about that. Just make sure the cage is large, has numerous and adequate basking and hiding spots. Don't be too shocked if you come home to find one snake eating another, not saying it's likely but it's more likely to happen than keeping them alone!