Im assuming yours is an eastern bluey, they certainly do house in groups relatively well, i dont do anything special just make sure they are similar size and have plenty of hides to get away from eachother and then monitor them. best case scenario nothing happens and they eat and get fat, they maigh ahave a few scraps and get some bite marks but nothing serious, but there is the chance that some serious damage could happen but it is rare. I have seen broken legs, lost tails even a fractured spine and one really nasty one where a male western had half his bottom jaw bitten off and all this months after introductions, so it happens but its rare and you can take steps to reduce it like keepign them well fed, plenty of hides and plenty of basking positions