I've done it using Bodyslide's outfit studio. It worked pretty well. I just made one body with tits that kinda looked like they were floating and saved the mesh, then made another with really low tits and saved that mesh. Then I loaded one in the outfit studio, then the other as an outfit piece. One pair would appear to be resting on the other and with no clipping on the body. Bam, four tits. You could do this as many times as you want. I'd upload a screenshot if my PC hadn't been struck by lightning a few weeks ago. Oh, also, HDT , BBP, and TBBP all still work on both pairs of tits, with no noticeable ass clipping.
