Is like keeping pairs, only different..
In order to keep pairs sucessfully you must have husbandry that allows for both to flourish singly, only even better, because they must be in the same environment. Next, both of the same species must get along or be able to avoid each other (lots of space).
Adding 2 of separate species together makes it more difficult, now the factors become more difficult, more space, more resources, more of everything to keep them happy.
Finally, keeping 2 of different sizes is hard, or sometimes impossible, simply put monitors are (in their own environments) their own worst enemy, why, they kill and eat more monitors of other species, and of the same species (different sizes) than any other predators. So now factors involving realestate to live on, resources to use, own, etc now become far more important.
So, if you are not 110 % on keeping one monitor alone, you should not keep 2 of the same species together, especially 2 separate species together.