I'm not sure how the OP has his pipes sized - but the way I look at it..... If the secondary pipe is taking 5% flow... a snail crawls into said pipe allowing that 5% to continue to flow with no issue - something happens to the primary drain - you now need that secondary drain to pickup the slack, you have a problem.
Do I think it's a likely scenario? No. Do I stand by that makes the secondary pipe a secondary and not an emergency, yes -100%. This is why 3 pipe setups are superior in every way.
The main has a gate on it. That gate is closed quite a bit to get full siphon. The main also usually has a strainer. Flow them out. Depending on your setup the secondary will flow 25-75% more water assuming same diameter, same bends.