I found this article that ought to work for both Windows and Macs. It doesn't address how to resolve any latency issues between the computer speakers and headphones. Let us know if it works or not.

https://thegadgetbuyer.com/how-to-make-bluetooth-speakers-computer-speakers-play-simultaneously/

Author of "Status 101: How To Keep Up In A World That Keeps Score While Buying Into Buying Less"