I am sorry to hear about the complication you have experience. You are wise in selecting your plastic surgeon carefully; there is definite learning curve when it comes to correction of symmastia. Generally, in my practice, this procedure involves internal suture “repair” (capsulorraphy) of the breast implant capsules along the cleavage area plus/minus the use of acellular dermal matrix. You may find the attached link, demonstrating many examples of symmastia repair, helpful to you. You will also find many examples on this website as well. Best wishes
Symmastia is very uncommon as you know, so even an "experienced" plastic surgeon may only have done a couple dozen in his/her career. However, the techniques involved are done with many other breast procedures, and these might include placement of acelular dermal matrix (ADM), creating internal tissue flaps, postoperative compression, etc. Find a plastic surgeon experienced in both cosmetic and reconstructive breast surgery.