[thenoblesunfish]

Good advice, in general. But there is a risk here, which is that you're encouraging people to do something which isn't their job.

Are you sure you don't want this person working on the things they are supposed to be working on?

Further, even if it all goes well, you're encouraging someone to do a more stressful job and not (yet) compensating them. Given the fact that you can't practically promote everyone you want, this risks your senior-with-junior-title feeling taken advantage of and prone to look for their title elsewhere.