Make one. Go to your nearest chanldlery and get a few meters of spectra and 3 pulleys, as well as 2 saddles. Also a pool noodle or 2 for kmart and cut them up.
Supsoulrider, are your 11' and 12' boards race type or cruise type? Please ignore my ignorance of everything not surf sup as that's all I have. As stated, weight wise shouldn't be a problem, but the o&e straps are a tight fit around my boards as seen in the photonand I'm guessing wouldn't fit around 2 race type boards. I may be wrong but you might have to get some loner ones if they're race boards??
Just buy a (cheaper) set of tie-downs. I bought a set from a bargain bin at one of my local retailers and when I explained what I wanted them for they even cut them down a bit for me with their hot knife.