You could also have everyone stand in a circle around 1 person, designate a block or punch or combo for him to execute, then have the others take turns attacking in a random order. Have the attackers kiai so the poor defender knows where to turn, start slow, then go faster and faster.
The defender has to utilize both hands in blocking, striking, and accuracy, it will work his balance and reaction time too. Switch techniques and defenders as necessary.
Plus the attacker gets some stuff out of the drill too, lotta fun!
If your students are good enough, you don't even have to name a technique, just have the defender defend himself against whatever comes his way!
Just an alternate suggestion, take it as you will.