Great idea!
I do that, I have also flown with other CFIG's and tend to make it obvious whom is flying and whom "may" take control.
In limited cases, the "friendly handshake" is replaced by, grabbing the stick, huge wiggle with a "my ship!" Is crude, but our asses are on the line!

Most "fun" to me was flying power with a towpilot/my GF at the time. Coming in on final in a C-150, short of field, stall warning going off, calm day, they were reaching for yoke, my comment, "Don't **** with the plane!". Hit the numbers, slight tap on brakes, first turnoff.
Stall warning was early, butt and feel said plenty,of margin.

Yes, knowing whom is flying is paramount.
With my mentor or others, a stick nudge or comment is a suggestion to do something else, but, no transfer of PIC.