If you google "how to carry two kayaks on a car without a roof rack", there are several articles on how to do this, generally with pool noodles or foam blocks. Sorry, can't endorse any one method over the other.

I am also a kayak camper, but at least for my kayaks, due to width, both wont fit flat on the roof/crossbars.  Plus if I have a kayak flat on the crossbars I can't open the rear hatch.  I have to use the J racks, but I don't have folding ones, so kinda a hassle to constantly take them on/off due to underground parking (work, visiting my parents condo...). I'll probably have to buy some folding J racks at some point...

At some point, I'll probably have to buy some folding J racks.