I've bought several straps over the past few years. A few of them were of the double-strapped variety, and they either shifted my bass into an unnatural playing position or scissored my neck.
A buddy has a leather double-strap, and he swears by it, but it was custom-made for him by a leathersmith who has since passed away. I guess the only way to get a double-strap to work for me will be to have one custom made.
Please let us know if you find one that works for you!
