Sorry if Ive repeated anything, anyone has said already but Ive skipped through the thread.
Seems pretty simple to me. I'd say your little beardy may be trying to go into brumation already. If your using a heat mat in a glass sided enclosure, your ambient temps may be quite cool, even though the probes on the thermometer are reading higher temps. Are the probes on the ground over where the heat mat is??
Beardies luv over head heat ie: high wattage white globe. They are diurnal, ie they wake up in the morning seeking warmth from the sun. I have depending on enc size high wattage normal white globes that are on during the day getting the spot temp up to about 40, with the immediate ambient temp at the hot end to about 32-35. Lights completely off at night. Once the temps start dropping into autumn/winter, they'l hide under things/dig holes etc to wait it out (brumate). A couple of mine are starting to do the same now, and it doesnt have to be a big temp decrease to trigger them into. I in the past have tried not to allow my small juvenile animals to brumate, ive kept the temps up for the first yr. Others have however let them sleep, as long as they have good condition.
Keep them seperate as your doing. try putting a divide so they cant see others, not sure if this is the problem though because at that size I find you can keep them together anyway till they get bigger.
Anyway, my two - bits worth. all the best