Aerofoil,
For formation flying, which is basically what you want to do, I would recommend that you remain within a mile or less from each other.
Normally when you are receiving radar service (VFR advisories or IFR) the lead aircraft is given a transponder code to squawk, and this aircraft talks for both aircraft. The second aircraft would either have their transponder on standby or OFF, and not acknowledge any instructions unless specifically requested by ATC.
You would contact ground, tell the controller that you are a flight of two, and request to taxi to the active runway. After both of you complete your runups you contact the tower and again, state your call sign and that you are a flight of two (what ever aircraft type) and are ready for departure.
When you are cleared for takeoff your buddy just follows you onto the runway and departs behind you (I recommend he not start his take off roll until you are airborne). Your buddy then stays as close as he feels comfortable (within a mile or less), and follows you to your destination.
As you progress along your route he changes frequencies when you are instructed (it's important that he monitors the frequency just in case you miss something or there is a problem).
We encounter groups of pilots departing together every weekend as they fly together going somewhere for breakfast or a luncheon.
Mike
NATCA FWA