So that should be straightforward then - the student (RHS) has FLT selected and the instructor (LHS) has IDL selected in normal flight. But if the student selects IDL as well then the instructor should be able to select FLT on his if the student forgets to reselect it after an auto?
I don't actually think the switch is a big deal but I'm sure some will get bent out of shape about it.
The issue could be regularly swapping from one type (with a twist grip throttle) to the 505 for training. If you jump out of a 206 after having done auto's and into the 505 then yes, you could go to wind it open only to remember it's a switch, however the 505 spools up so fast to flight, you could get away with it until late into the flare.
The 505 is far more complex than it's predecessor (and the 44 etc) even though it's still a single.
If you're instructing on the 505, maybe you need to brief yourself before starting and as always, good CRM and TEM would hopefully pick up any potential issues.
With all that said, I can see how easy this happens. You have a long day teaching on the 22/44/206 etc and then at the end of that day jump into the 505 to do some auto's. Muscle memory is a twist grip.