First, lay the full length pipes on your trailer, but don't attach them yet. Then, put the boat on them. Measure and cut the pipes long enough that the entire boat is supported, plus a little extra. 7' or so will work for the PA-14s. I used 8' on my double jet ski trailer because that distance worked for the cross beam supports, and my trailer doubles as a utility trailer with a plywood floor.
Once you have the length determined and cut, again lay the pipes unattached on the trailer and put the boat back on them. The width between the PVC tubes is determined by the size of the PVC, and the type of boat you are putting on them. Most boats have a different width between the front and rear, and the length of the pipes will also figure into how far apart they have to be. Move the entire rig around until you have it sitting exactly where you want it insuring adequate fender clearance, then mark the boards under the PVC with a Marks a Lot. Just make sure the boat is sitting securely on the PVC. Some say to let it sit for a while before marking. Once you have the marks, remove the boat and mount the PVC pipes securely on each end, but not in the middle because the boat is wider there and will spread the pipes. How you mount them is determined by your preference and how they are attached to what is under them. Regardless of how you mount them, be sure to check often to insure the nuts or lag screws haven't loosened from road vibrations.
Repeat the process for the other boat. You will also want to bevel the rear of the pipes so they don't scratch the boat as it goes on and off the trailer.
Finally Retired, Fish Beware!
2013 Hobie Pro Angler 14
2012 Hobie Pro Angler 12
2001 Hobie Hank Parker Edition Outback