Nobody drafts 1st round QBs in back-to-back years unless the first one is terrible for a reason. To get multiple 1st round picks in return for a player that player needs to prove they are a franchise QB for a white. That means they need to play, a lot and consistently well. So you will need to start the QB you want to trade, hoping that the backup you aren't playing will be as good or better, but you don't know because they don't play. And if you do this and it all happens to work out then you have to explain to the team/ownership/fanbase why you didn't play the better player all along! And the time you spent showcasing the lesser QB that you want to trade you have 1 less talented player on the roster because you spent that pick on a backup QB. GMs and coaches have to prove themselves yearly or risk losing their jobs, so they are not going to risk current success on a gamble like this.