I used that method on my cat for several years with no problems, but I’ve since reconsidered given that crossbeams are not meant to take heavy loads perpendicular to their length. I think it’s better to tie a bowline on one end of each dock line, put the bight around the deck cleat, run the bitter end through the morning ball pennant, then back to the deck cleat, secured with a cleat hitch over the bowline bight.