Yes, very serious. I teach this for a living, remember. Side by side you'd both be on the floor, secondary rider hanging back you usually have the time and distance to escape without gathering each other up, and the secondary rider has a full vie if their colleague on the inside. The riders should also be communicating with each other 're hazards, the primary rider calling out ahead and secondary to the sides and rear, though neither should stand on ceremony of they spot something outside their own arc.
Ride parallel and not only can you not see what the inside man is doing without turning your head 90 degrees, but if one has to move suddenly to avoid an obstacle they're highly likely to turn their friend into an ex friend. If you're both roadies you'll liable be travelling quick by bike standards, will have less time to react and have more kinetic energy at play when it goes wrong.
Even trying it deliberately (and we get our students to t it deliberately in the safe environment to prove the point) its all but impossible to take out your partner if you're staggered. If you're parallel it's difficult not to do so when it goes breasts skyward. It's an internationally used technique, written in the blood of less careful riders.
