I have very little experience specifically with collared lizards, but with captive animals in general, I'm concerned only with whether they have constistent appetite, and look happy and healthy.
Beyond that, if one lizard is growing more slowly than the other, I will *try* to repress the natural concern that something's wrong. Granted, I'll probably be more concerned that the smallest lizard always gets something to eat, and that it isn't going to be picked on excessively.
I remember with some kinds of cichlids, if one fish grows too slowly, its going to be a problem eventually, since it will eventually get beat up too much, lose it's appetite and in order to be humane you'll have to find it a kindler gentler home.
I get the impression that being of comparable size isn't so much the issue with collareds, but competition with a male might be the larger issue here.
Again, I'm still under two weeks keeping collared lizards! Take what I say with a grain of salt, or if you're on a low sodium diet, just skip it all together.
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1.3 Collared Lizards
90 Gal Aquarium
Minneapolis, MN