Hi Andrew,
It's a piece of cake. You need 4 blocks of high density polyurethane foam about 3 inches thick. The sort used for furniture cushions is ideal - cut up the old sofa or buy new from Dunelm Warehouse, etc. I have seen car tyres used but they are heavy, ugly and can leave black marks on the boats. Strap the bottom boat to the trailer securely before putting the second boat on top. Put the cushion pads on each beam housing so that they are mostly in front of the beam. Turn the second boat upside down (best with 4 bods) and place carefully on top of the cushions such that the top boat is about 8 inches further forward of the second boat and that the beam housings rest on the foam blocks. It is important to be careful of the main sheet block and traveller as it is easy to bend the traveller cleating arrangement on the lower boat by putting the top boat too far aft and ontop of the traveller. It is best to take off the mainsheet assembly of the lower boat.
Now tie the top boat firmly to the trailer. It is worth tieing the two boats together at the rear beams to stop the top boat shooting forward when you brake.
The tyre pressures depend on the size of wheels. I used to use 55psi in small 8" wheels on my trailer but bigger wheels take less. If you are using the trailer you bought from George I think we used to use about 45psi in that when 2-upping. Maybe George can remember. It has been use 2 boats -up many times.
Cheers
Bob
PS Is this so that you can bring 2 boats to the Nationals?