As you have said, the chance of meeting accidentally is remote. I can only suggest that you meet at the first significant landmark, the Cutty Sark (just over 6 mile). Here there is a sharp left turn as you round the bow of the ship, you could arrange to meet on the extreme outside of the bend at this point. Alternatively, choose one of the mile markers after the two starts merge, and wait there, but please make sure you are not in the way of other runners. If you have trained together, hopefully you will arrive about the same time, although it will be pot luck how much each of you are held up at the respective starts.
