As some others have mentioned in other similar threads, both SLI and Crossfire are kind of dying out.
In general, in titles that support it, you will get like 20 to 30% extra out of the 2nd graphics card. If there is no support for it by the title, you get no boost to performance or negative benefit.
Also as side note, physical size of screen doesn't mean anything, it's the pixel count, which at 42 (if it's not TV) is pretty likely the UHD/4K
which also means that as single card, you would need 1080Ti or something along that line.