I don't know about monitors in particular, but I was always against putting 2 different species of reptile in the same enclosure, for various reasons. The biggest reason is, if 2 species are from 2 different regions, similar ecological conditions, but different regions, ie, tropical and the amazon vs the congo.
Each species will have its own variety of micro-organisms that it is used to, or that it benefits from. And if they happen to spread to each other, the effects could be very adverse.
Then there is the possibility of aggression, one being a more dominant species and potentially hurting the other. Even if no aggression is shown at first, think of how one day your monitor might be fine with handling, but the next its first response is to puff up, hiss, tail whip, and bite you if you don't get the message.
It just doesn't seem like a good idea to me. I wouldn't do it with any reptile, let alone monitors. Varanids are a whole different ball game than beardies and geckos. I've never had one, probably for that reason. I never felt I was ready until now, and currently I'm deciding which kind it is I'd like to purchase.
Best of luck with whatever you do, but for the sake of the animals I wouldn't recommend it.