This. Having one attacking and one defensive FB is a recipe for trouble. Teams have done it, of course, but it makes it so much harder for the CBs to run a good defensive line when one of them is having to offer so much more cover than the other.
We should build a team around our existing strengths, that's obvious. We have two of the best defensive fullbacks in the league, maybe even the best. So play them both, and let that extra defensive work give us license to play a more attacking midfield, say Fred/McTominay behind Pogba and Bruno. We already see Shaw and AWB stepping into midfield to make really useful defensive interventions, so build that into our strategy. The attacking resources you can then field in one team are just frightening.