That's fie for your personal use, but for selling images the buyers are looking for insane detail, down to the patches on the pilots' arms.
LOL are you kidding with this post? You're not going to see '...the patches on pilot's arms...' while shooting two aircraft in a crossover for a host of reasons that have nothing to do with focal length.
Seriously. They want to be able to do an A3 spread cropped to the cockpits.
Do you have any idea how tightly you can crop 50mp and still do A3?
That said, for a really detailed in flight cockpit shot you have to be close to the aircraft and the aircraft has to be angled so that you are looking into it. So basically a relatively slow, nearly overhead turn. When such a turn occurs anything 400mm and above will be enough. Hell, you might get away with 300mm. If it's not overhead then you are not going to "overpower" the distance with 600mm, 600mm x1.4, or even 600mm x2.
That's the dirty little secret with airshows. Having an idea of where a plane is going to do the thing you want to capture, and walking across the base to get as close as possible to that location, counts for more than focal length ever could. The relative magnifications between an aircraft passing over and an aircraft down flight line are easily an order of magnitude greater than the focal length range in Canon's (or anyone's) telephoto lineup.
In any case, you're never going to be that close to a crossover unless you're military. The maneuver isn't as dangerous as it looks, but it is still dangerous and it is not performed over civilian heads.