I saw a temporary covering therapist at one point when my therapist was away for some months with a family emergency. It was a useful stopgap, and somewhat odd, and would sometimes leave me a bit bereft. But it was definitely a net positive as far as keeping myself on track with dealing with my issues, and I think I would have been feeling even more bereft without someone to vent to in that way. In my case things were more geared towards support and keeping a holding pattern with the substitute therapist, rather than insight the way things are with my usual therapist. Which felt appropriate at the time, though I might have needed something different if it had been more than a couple of months.

In your case it seems like an advantage that you're seeing someone you're familiar with already? So at least they'll know your basic history, even though you're usually just seeing him for med management?