Keep the .404 with these large bars and saws, it handles the load substantially better than 3/8, less stretching, less sharpening, for just a bit more kerf width. I have an 881 since 2021, hasnt missed a beat during all this time, 99% milling with a 48” setup, running 25:1 mix. Very happy with my purchase. As for the double ended bar and saws, I wouldnt do it. The bar is expensive, you need a helper etc. 2 saws = 1 more things to keep on eye out, servicing, problems etc.
Buy any one of those 2 big saws and you’re set. A few nice slabs and they paid for themselves. I’d get the 881 over the 3120, the side chain tensioner is nice, and it has more grunt once the exhausts are opened up the same.