In order for Trey Lance to have legitimate trade value, he would have to perform at a high level for a significant amount of time. I would say three games.
It would have to be a situation similar to Cooper Rush last season. Rush started five games and the team went 5-1 during that period.
Of course, in order for this to happen, Dak would have to be seriously injured long enough for Lance to start those 2-5 games.
There is the possible scenario it would backfire with Dak out long enough for Lance to establish himself like Dak did when replacing Romo in 2016. It's not an impossibility that Dak would be the one traded.
In my opinion, this is the best opportunity for the Cowboys to have two starting caliber QB's on their roster. t's very rare when a backup QB has the skill set equal to a starter..