Just a FWIW .. .. .. a few years ago, my Woodmaster 718 developed a vibration. It is a 2 "matched" belt system. Just to check it out, I pieced together some link belt pieces, and installed it in place of the
2 originals. The vibration disappeared, and I never have either replaced the "matched" set or added a second link belt. It has been running just fine on one link belt ever since.
Now, if you think you can ever load up the arbor on your saw to exceed that of an 18" planer,perhaps it is correct of you to go buy a matched set and be done with it. Myself, I'd try one or two, or even 3 link belts without hesitation. Look at it this way .. IF you were to install 3 link belts .. would this not be a likely scenario. The shortest belt will begin to take the load .. .. IF the load gets great enough to stretch the first belt, the second one begins to grab. IF they both can't pull the load (RIGHT !!), #3 kicks in, and what, exactly have you lost ??
All the theory and logic in the world don't mean squat when you just try something and it works .. WITHOUT any negative side effects. What would be the worst case scenario for simply trying that which you propose ?? NO HARM .. NO DAMAGE .. NO PROBLEM .. go for it, and let up know how well it works out for you. Be sure to let us know how much it slipped !!
As for your original question .. buy 6' of link belt .. if you get 3 out of it, great .. if not make 2 .. run them as is and get the extra foot of belt to make up #3 if you really think it's necessary.