I made a boiler plate version of the Australian Aero-High sounding rocket to test at my rocket club launch Sunday afternoon. It is a gap stage model (the booster engine separated by a few inches from the upper stage.) with internal venting to allow the flames from the burnout of the booster to ignite the upper stage.
It worked perfectly, placing a bright orange cloud (Yes Brooke, THAT orange!) satisfactorily high in the sky. The insides were a little scorched from all the flames, but I believe that can be taken care of in the final model with a healthy dose of epoxy.
The event is B scale altitude and I used an A10-0T booster engine and an A3-4T upper stage engine to add up to the total impulse of the B engine category.