We use the straps that came with the swings. we bought both swings used for like $15. For double swings we use aluminum snap-shackles to hook the swings together. 2 is better than 1 because it stops side-to-side motions. The babies love it. The adults are happy too because it keeps the kids up off the floor where they fall and trip a lot.
Here is a video of the two-swing setup. Single is just like it except hung in middle between the speakers. Depending on sun and activities, we face the babies either backwards or forwards to suit.
https://youtu.be/t843wnMg79s
