It's certainly an unorthodox move. If this has been done before, it hasn't been done often. Legendary coach Bill Parcells had a saying in his time with the New York Giants. "If you have two quarterbacks, you have none."
Quinn hasn't played a snap since 2009 when he was with Cleveland. Quinn led the Browns to a win over Kansas City in Week 14 of that year. The week before it, his 6-for-19 passing and 90 yards were enough to beat Pittsburgh in one of the bigger upsets of recent Steelers memory.
The question is whether the Steelers can seamlessly adjust to two quarterbacks with such differing skills. The aspect of their game they have the most in common is the fact neither of them are very good passers. Quinn has a stronger arm, and there are obviously concerns with Tebow's decision-making, so the move could be because the absence of Tebow on passing downs is more valuable than his participation in them.
Denver is mostly a run-based team that wins on first and second down, setting up third and short situations. They will need to complete passes, but much of their game plan is likely scheduled for their running attack. Their defense is strong enough where success on third down and 10 points could be enough to win.
It remains to be seen whether Quinn is the guy to do it.