As of right now, there exists a range where you can cast on both dragons without being in range of one debuff. It's a relatively precise positioning, but it exists. The first couple times I tried, I ended up getting stunned after the swaps due to a change in where the tanks are tanking each dragon after the swap.
What I ended up doing to prevent this was hug my new dragon a little bit harder(moving out of cast range of the other dragon for a few moments), then slowly moved toward the far dragon. When you first enter the cast range of the second dragon, you are in range to cast but close enough to receive the debuff
Edit: In theory, you can be out of range of both debuffs and still be able to cast on both dragons. In practice, I doubt it's useful at all, outside of emergency situations where you accidentally refresh the debuff you're supposed to be dropping