As tunes said, for an actual quarantine you need them in separate rooms, as well as make sure you wash between handling, ideally also change clothes as well if the new one ends up being sick. Things like URIs and ringworm can spread in the air and on surfaces, and things like giardia can be spread as far as the poop travels. If you have to have them in the same room at least try to have them as far apart as physically possible (I realize not everyone has another room), and no out of cage time where they get any closer to each other's cages. Aside from illnesses, the other reason for quarantine is to allow the new chin to settle in to the new home and feel at home. Without another chin around right away it can also be easier to get an idea of what normal behavior is, as well as for you to start bonding with the new chin.