I would like to see it stall before Vmc though

 

Was Should be: "Counterrotating twins don't stall in vmc demo before reaching vmc as long as the density altitude is low enough." (Thanx Rickair.)

You should fly low enough to get to Vmc before you get to Vstall, yet high enough that if the student shoots right through to stall you can recover without hitting the ground. If the prevailing density altitude is high enough you will encounter a stall before the Vmc and it would seem prudent to check it before doing a Vmc demo.

Say, normally the aircraft stalls at 58 and Vmc's at 62 at sea level, and you choose to do your demo at 3500 msl. The airplane Vmc's at perhaps 59 kts and all is well. But it's hot and humid the next day and you are at the same altitude, and Vmc has dropped to 57. Student and instructor both look for Vmc and lo and behold they are in an inverted spin before they know what happened. At 3500 msl you don't have long enough to get out of it.

-Cub