As someone already mentioned having two beardies in one tank can be an issue, I can't tell for surw but the other one looks larger if thats the case he will most likely be the dominant one, taking all the food and the best spot for basking. If the smaller one is not able to bask he won't be able to digest his food meaning he won't be hungry he also won't be getting any uvb whoch means he won't be able to absorb calcium. I would seperate immediately set him up in an enclosure with his own lights, see if that helps, if you can't get another tank you can use rubbermaid or steralite tubs at home depot or any home goods store, they are usually cheaper than tanks and hold temperatures way better, 100L is a good size. If your little one is in fact sick you want to seperate regardless ge might have something contagious that can spread to the healthier one.