Okay. Let me impart some advice if I may: shrinking down, while unsettling, is oh so normal within the first day or two. Gigs and I'll assume it applies to Mertens (reefvette can confirm) since they are so closely related will never deflate unless on death's door or something REALLY wrong. However, this is not alarming behavior WHEN ACCLIMATING. It's to be expected then. I will say that if every day or two it deflates.....not normal even during acclimation.
I remember bringing in gigs and having them deflate during the first week while acclimating (and me getting worried - thinking some parameter was off or something chemical going on), which would lead me to make sure my pH was "perfect" and make Mg adjustments, SG tweaks, big water changes.....all a bad idea. The only thing you should focus on is STABILITY around NSW conditions - which you should already have if you are keeping gigs and SPS with any kind of success. These carpets will adjust to what conditions you have (assuming NSW), but making all these tweaks in a response to some behavior observed in any of your carpets is a bad move. Make sure you are compensating for SG fluctuations due to evaporation.....that's one thing I noticed new gigs are really intolerant towards (anecdotally, at least IME). Once settled, they are rather tough. Run carbon, be cool.
On seperating your carpets: I saw the same thing happen to mine, where one kept creeping with its foot to crowd the other away. Beware, once I moved the purple, the green would wander in the direction of the purple several inches every night no matter where I moved it. I am convinced it was hunting it. This went on for around a week. I don't know if yours will do this or not - but you better believe they can sense each other in such small volumes as our tanks. Finally moving them to opposite corners with strong flow running between them has kept them each to their own spot. Cover your powerheads or turn them off during the night (which is when they will move if they are going to). Having to turn off powerheads is a bad idea though, as they won't be able acclimate to their flow (which may cause more moving by itself).
HTH.