My first impression on hearing about one-finger double stops was "that's impossible," but it didn't take too much practice to get fluent with them.
The very first double-stop chord I got down before moving on to the 6/9s was a simple 1st position A-minor:
(This is the fingering suggested throughout Unaccompanied Django.)
This and the first position E-minor are the simplest double-stop chords you can play. I used them to convince myself the technique was possible before moving on to fuller forms.[/i]