When working with behavior, there is no one answer, there is only temporary decisions.
Its called "working" with animals. The reason its said your working with with them is, they constantly change. They are living animals for gosh sakes. There is no set them up and go.
When keeping monitors together in groups or pairs, you must understand, it does not have to be permanent(this word does not exsist in nature) And because its in captivity(small cage) You as a keeper must intervene when necessary.
Monitors are very very behavioral. They form their behavior on their current conditions. If they are solitary, they then trust being solitary. If they are raised in pairs or groups, they then trust those they were raised with and trust and perfer the group.
Then one must understand, time and age, effect all behaviors. That means, behavior changes with age. This is constant with all living animals and monitors are no different.
So if you have young animals, they tend to be more socially pliable. If you have old animals, they tend to be set in their ways. All of this is common sense(that many academics do not have).
So with some understanding of the above(behavior) only you can make your decisions. This is the gamble you took when you decided to work with animals. Then the greater gamble when you decided to work with Croc monitors. Mind you, croc monitors are no different then other monitors, and in fact, slighly more forgiving, but they do have huge weapons to inflict lots of damage and do so very quickly. In other words, you paid the pipper, now you have to listen to your own music.
Sometimes you get lucky and things go well, for a while, the question is, whats awhile. Cheers