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.
