I am extremely strongly against this. No other companion ends up with others, so it would be unusual for these two characters to have a unique ending. There is also the small issue that if you are doing a generally good playthrough, Halsin tags along whether you like it or not. Halsin x Shadowheart would add another unwanted element for players who don't particularly like Halsin but are running a good-aligned campaign. If they want to implement Shadowheart x Halsin, they must also provide a way to leave Halsin in Act 2.
Wyll and Karlach end up together, if they go to Avernus in the end.
I don't see Halsin and Shadowheart either btw, she was much more in awe for Karlach and even would make sense with Lae'zel, but you can get Halsin abducted pretty early and 'botch' the persuasion role, if you don't want to deal with him. I mostly just ignore him in camp.
Generally I don't care, who ends up with whom, but I kind of do ship Karlach and Wyll tbh.