It may be a high arousal situation for your dog, coming in from a walk. See link for help with this problem (article from professional animal behaviorists).
This may be caused by your dog feeling “his” house is for him only, and coming in from the outside to his domain he does not want to see anyone else there.
When you come in, is the lab waiting at the door?
When you come in, ask your roommate to have the lab somewhere in the house not readily visible from the door you and your heeler are coming in. this way, your dog can come in to a quiet house and not feel threatened.
Another option, if your schedule allows it, have you and your roommate take the dogs for a joint walk, so neither is home when the other one comes in.