By gut instinct, I don't think that's bad either. It would be like just have one really thick 5.0 tube light, it seems to me, not the equivalent now of a 10.0. If you had a UV meter we could confirm if the output still remains the same, which I don't know.
However, if you decide to not do it, you can get a 5000k light fluorescent from any home improvement store and put it as the second bulb, and then you still have your second 5.0 as a replacement in 6-7 months' time. This would still add brightness and help the plants out but not output any extra UVB.