I use front and rear friction drive w/43cc 2.2hp Mitsubishi engines on "The Dragon Lady". The rear engine has an ADA expansion pipe. I removed the flame arrestor on the front engine.
"The Iron Dragon" has front friction drive and rear chain drive with same engines. Less friction resistance, quicker and faster than the "Lady".
Both engines work independently of each other. They are mild-mannered and I've never had any nuts or bolts loosen and fall off from vibration.