Formation Flying
As a pilot who regularly (several times a year) flies very long distances (getting on for 1000nm) in formation, including a channel crossing, I am reading this thread with great interest.
I very greatly enjoy formation flying, and when done properly it is a major win-win for getting multiple aircraft to cover a large distance - the most experienced guy, or at least the one who knows the route best, leads and the no. 2 (and 3, 4, ... if they exist) only have to focus on their station-keeping. The minimum equipment requirements for the non-lead aircraft is lower (esp. no need for transponder), there is no need to be familiar with the route (but they will learn it for next time), R/T is simplified, and **crucially** it provides an additional safety margin in the event of going into the Channel - faster alerting of D&D, relaying lat/long of survivors, and can be helpful with overland situations too.
BUT - and it is a very big but indeed - formation flying is a very specific flying skill which must be learned. It is highly dangerous to engage in DIY formation flying (as someone else described it), whether formal close formation or loose formation. Crucially - and I disagree with some previous posters over this - it is a myth that flying in loose formation is more safe for novices than close formation, for two reasons. First, in close formation you have no option other than to be highly and minutely focussed on the aircraft you are formating on. As it requires less concentration, loose formation can lead to a lower level of awareness of the location of the lead aircraft - which is still quite near!! This could be - and has been - a recipe for disaster. Second, in loose formation you are naturally going to be less focussed on navigation, situational awareness, etc. Again I fear that this can lull the non-lead pilot into a false sense of securiy and can impair the process of responding to a changing situation. Lastly, in the event of comms problems in formation (which I have experienced several times) you need to be within visual range of the other aircraft to pass hand signals.
My advice is pretty clear - get some proper training in formation flying from a reputable outfit, which will cover not only joining, leaving and station-keeping, but also focussing on "formation management" issues: planning, briefing, hand-signals, dealing with emergencies, formation leading, etc. etc. Once you have had this training, go and enjoy it - formation flying is imho one of the most rewarding aspects of flying, and one which very few civilians get to appreciate properly. Until then, go as two singletons - it's safer overall.