Is it ok to just use one bow & stern line for 2 kayaks on a roof? Or do I really need two sets? I really suck at tying/untying knots...
One set is enough, as long as you can
see that the fore/aft and side to side triangulation forces are keeping both boats steady.
The knots needed for end lines can be very simple. I use the truckers hitch for tensioning. I often use a small carabiner tied to the line for one of the connections to the vehicle.
That’s What I Use…
…carrying two kayaks unless I expect really heavy winds. In that case, I tie the first bowline to each as usual, then add another for each boat that crosses to the opposite tiedown point - think of the end result as looking like an X within a box.
I think of noselines as a backup safety system - they are there in case anything securing the boats to the crossbars or something on the rack setup fails. The straps holding each boat to the fore and aft crossbars are what really secures the boats to the rack.
If you want to avoid knots, use a long strap, same type used to secure the kayaks to the car but longer for stern tie down. I use one for both kayaks. Straps may be a bit thick for the bow tie down, there are systems for sale that have two hooks, one adjustable.
Or just learn to tie a few good knots. Here is a great online animated website: http://www.animatedknots.com/
You got to learn the bowline and truckers hitch and maybe double half hitch. 3 easy knots is all you need.
I use what I call the double truckers hitch.
I tie off to the bumper or what ever to the front or back of the car, come up through a loop on the first kayak and make a truckers hitch loop low down on the rope near the bumper then go up to the second boat and then back to the same line not so near the bumper with a truckers hitch.
I am currently using two lines because the boats are very different lengths but the double trucker would probably work with these also. I will try to take a photo next time I have it setup.