Leaving aside the suitability of cohabbing in general for the moment, what is your essential research question for this project? It seems ill-defined from what you've shared so far.

Is it behavioral? Will you be observing the effects of the other animal's presence on each other's behavior? If so, the lack of a baseline for behavior from the individuals will be a problem.

It sounds like maybe it's related to husbandry? If so, what is the research question that benefits from the presence of both? If it's merely to prove that you can create a habitat that meets the needs of both, that's not exactly a research question and also doesn't essentially benefit from the actual presence of the animals. There's a number of questions related to husbandry that would make more solid research questions.

For example, you could observe the effects of UV light on diurnal snakes such as garter snakes that are not considered to need it but has some arguments over if they benefit. For this, you could have two snakes in two cages cages, smaller than you'd need for cohabbing the lizards, one with the UV light and one without for the first six months, and then switch which one has the UV light and observe behavior in each set-up and behavioral changes with the light swap. This, like the previous proposal, is obviously limited by the small sample size, but has the possibility of suggesting something of interest.

Another possibility would be documenting behavioral effects of various environmental enrichment on snakes or lizards. This could be done either by adding various enrichment for periods of time and observing behavioral changes, or setting up one animal in a planted vivarium and another in a minimalistic tank among many other options. This would provide data on the responsiveness of reptiles to various (positive) stimuli.

You could also investigate the ability of reptiles to learn via food motivated training. It's easier with some lizards, but there is a woma python out there that has learned to open a puzzle box for food, my own garter snakes are mostly target trained although one does a better job, and turtles have been found to do a good job of mazes once they were finally tested with better ambient temperatures. You could get two similar species and compare how well they do, you could track what stimuli one specific individual of one specific species responds better to, you could track what conditions affect how well an individual does, lots of options really.

Basically though - what are you trying to find out and why is this specific set-up the appropriate choice?