You should use the tube/DLR station for the appropriate start,it is possible to arrive at the opposite station but will involve a large detour on foot for one of you to get to the other coloured start area.
Greenwich park has security on the gates to the runners area and if your start colour does not match you will be refused entry,and pointed towards the detour for your correct area.
If you wish to run together simply arrange to meet at the side of the road at the point where the routes meet at a suitable landmark.
If you must meet up before the start the church which sits at the top of Blackheath at the blue start area is an easy option.
Hope this helps and see you at the start 
Edited: 12/04/2012 at 09:59