A teacher got me practicing double stops; alternating m3rds/P5ths, M3rds/b'd5ths, etc. as a way of ingraining the correct hand/finger spacing. He had me practicing both stationary double stops and shifting double stops, all up and down the fingerboard (shifting a 5th double stop while moving the hand/arm a m3rd, M3rd, or P4th, for example. This works both in the lower register and in thumb position to make intonation (within the hand position) automatic. Once the stationary double stops are "grooved," the shifting double stops help chill out shifting confindence.
When I started using the open hand fingering, I started practicing double stop combinations within that m3rd spacing (that the open hand or extended fingering provides; one finger per half step).
In thumb position, especially if you are using an open handed thumb position fingering (some combination of half and whole steps between fingers). the range of double stops gets pretty wide; from a perfect unison (hopefully

) to a m7th.
Anyway, I still use double stops every time I practice; for hand position, for strength building, and conceptually, because it helps me ignore the strings when I play single notes. Practicing double stops helps me focus on tapping into the fingerboard rather than worrying about string tension.
I don't know if any of this makes sense, but I found that by trying some double stops every day, in whatever style of fingering one uses, in any register, it cools out a lot of problems (at least, it does/did for me).