I've found a good exercise to ingrain double-stops "melodically" is to practice your scales with a 3rd, 4th, or even 6th above the scale degrees. 5ths are easy, just a "barre" finger, but they are a little too strident on the ear.
This helps learning the finger "forms", or patterns, since they repeat. Definitely installs muscle memory for genres like jazz, country, bluegrass and folk. Beware, once you start playing double stops, it's hard to stop (no pun intended)! But, as a comparison, a harmonized vocal duet is so much prettier than a solo. Good luck...