Re: Bill Dixon, the 7-tette (1964, Savoy) has Hal Dodson and David Izenzon. Apparently Izenzon had a lot of trouble with his part, though I'm not able to recall why. After all, he was a competent classical bassist back then. The score for "Winter Song" is pretty meaty - Down Beat reprinted it at one point. There are also two bassists on Intents & Purposes, Reggie Workman and Jimmy Garrison. Dixon often liked to use two basses in complementary textural roles, which I assume was a way to "spread out" the sound, especially when working with a small group.
Brötzmann: Kowald and Niebergall on Machine Gun, to name just one recording. They were also two of the bassists in several Globe Unity recordings.
John Carter/Bobby Bradford: Self-Determination Music had Henry Franklin and Tom Williamson on basses to excellent effect.