Its very common on autocross layouts while playing online, whats happened is that both cars have joined at the exact same time. Thats usually due to lag, because LFS would normally prevent that. Basically you joined inside someone elses car, and the collision detection went mental

I reckon, if it was replicable on demand, it would make for a very interesting game mode, where you're in a team of two, and you have to set the fastest time and must cross the finish "conjoined" to score a time.