Fuji, thanks. That's the sort of response I was looking for.
So far it's just a hypothetical scenario, but you can assume that both pilots are qualified enough to make the trip on their own if need be. It's not intended as a cross-channel lesson.
More specifically, I'm thinking about the scenario where we would have three or four aerobatics pilots who all want to participate in an aeros contest a significant (two hours or more) distance away from our home base (Rotterdam), but the aeros plane itself can only take two people and virtually no baggage, and has a very limited (barely sufficient) avionics stack for long-distance touring. The other plane then would be an IFR Warrior or Diamond or something taking the other two pilots, the bags and such.
But don't worry about the exact scenario. Just interested in the technical/legal/ATC aspects of formation, or better, "in company with" flying.
Fuji, what's the distance between the planes if you fly "in company", what's the position of the chase plane and what sort of elements do you discuss pre-flight?