I'm a PhD student, writing my project proposal and also a junior teacher in the same university. An ex-student asked me if i would like to co-supervise him in his master thesis and perhaps suggest him a topic of research. It comes in my mind to include him in a task of my PhD project, which might be further explored by him, being also helpful for me. The other supervisor would be my own supervisor. Would this be ethical?
I'd consider that an excellent opportunity to get further into research, and gaining a pair of hands in your job (even more, as the advisor will also chip in). As long as everybody is happy with this, particularly the master's student (who clearly is on board, as you were approached in the first place), I don't see any possible ethical problems.
But you'd have to check carefully that this doesn't run against any local regulations.