This has become a growing issue in our household. My son just can't wait to drop his latest insight or question on us, even if Bee and I are talking! We've been telling him to wait until we're done talking, but he just can't seem to wait. FWIW, he has trouble with delaying gratification in general!
Not sure how to make progress on this one, other than helping him learn how to delay gratification and also just reinforcing the message that it's not ok to just interrupt two people who are talking.
If your LO has something to say, do they wait to say it or interrupt?