I'd suggest just watching them very closely and disagree with the staring behavior whenever / where ever it happens. Eventually, you won't have to monitor it so closely. They'll unlearn this "habit" of sorts. I would probably guess that the excitement of company also triggers their issues. Some dogs never become "best buds" but they are a pack and their DNA will force them to come back together always. A pack will always come back together --- regardless of the scrap/fight/issue.
If their trigger is especially quick, having a squirt bottle at the ready is what I used on Bo for this very thing. At about a year old, he became Bea's arch nemesis demon brother -- and it wasn't fair to Bea to be hassled so relentlessly and so forcefully by her little brother -- who outweighs her by 35 lbs. Problem resolved. Bo doesn't do that stuff anymore for the most part. Any current issues are easily corrected verbally. (I use "ACK" for my disagreeing sound) Iron clad consistency --- and fairness when using the tool --- is key to success.