While queuing for M+ dungeons, I needed MG Workshop and Lower Karazhan and I had to keep clearing my input on the search field and changing the dungeon title in order to queue for the other one.
My suggestion for this problem would be to allow the user to filter a total of 3 dungeons max at the same time while typing in the search field, and after they choose for example, Lower Karazhan, it would appear as a small filter chip with an X symbol next to it so they easily clear it.
After selecting the 3 dungeons you wish, you can press enter and then you would be able to see results for multiple dungeons, instead of having to switch constantly.
I believe it would immensely help users, it is not something that could be exploited and code-wise it could be implemented fairly easily.