It sounds like you are doing the right things. There should be a sports bra that gives you good support and limits tissue movement and stretch, without having to wear two of them. That's usually a sign that the fit or support of that particular bra isn't optimal.
In addition to Title 9, ChampionUSA.com has a good selection. Specialty bra stores also carry good sports bras by Panache, Anita, and Shock Absorber.
The bra that gives you the least movement is the best, but obviously not so tight that you can't breathe! Check title 9 sports website or catalog. They have an impressive selection of sports bras, even for large breasted women and miminal movement (each bra is rated for this). I am sure there are other similar stores, but this catalog and wide range of choices was impressive.
Basting description it sounds that wearing to bras is providing adequate support. You're correct that you want enough support so that the breast do not bounce and move a lot during exercise. However avoid 1 constrictive breast that is so tight it is uncomfortable.
Basically you want some good support to dampen the bouncing up and down of the implant. It is this jack hammer effect that can cause stretch deformities.