If Mancini and Santander are both traded, it just means they can call up Stowers and rotate the DH spot more easily. Adley is already getting some at bats there, Mullins and Hays could get half rest days playing DH, and if you want to call up some of the IF prospects it gives you more flexibility,
I would trade Santander before Mancini though. I just don't think the return on Mancini is going to be anything of value. Santander is a good player with years of control left, so you're more likely to get a return that helps the team, and it's a position of excess