That wouldn't work at all. They're two entirely different goalkeepers, so you'd either be forced to significantly change tactics from one game to another (always a bad idea), or play tactics that work for either keeper and get poor performances from the other.
As I said in another thread, given De Gea is not a high-quality keeper in any aspect anymore, I would tend to agree with people arguing that he should be let go and United should get two new, stylistically similar goalkeepers in - for example Verbruggen as a starter and someone else equally cheap as his back-up/competition. That's affordable, frees up a lot of wage space, and not more risky than keeping De Gea between the sticks (who holds back the team tactically and has even become statistically average at shot-stopping).