Dear suziesuzie,
This is not a technique that I use, although I have seen results using this technique and they have been good. The problem that I have with it, is that the results that I have seen have been on patients who have a reasonable amount of breast tissue already - the sort of patient that I would do a subglandular placement of the implant.
In my practice, I use the submuscular placement for people who have little breast tissue. My worry with the split muscle placement is that, if used on patients with little breast tissue, there may be a ridge half way up the breast where the muscle split has been performed. I am sure that the procedure is safe, but if you have concerns, it may be worth getting a second opinion.