T-junction in breast reduction with the anchor scar. How to prevent a separation in there? (Photos)

The T-junction and how it is managed is a technical issue that we surgeons deal with and prevent during the operation, and is not your role and not really under your control. Your plastic surgeon would be the one to prevent this during the surgery since the technical aspect of the operation as performed by the surgeon is the major factor here. There may be some do's and don'ts that your plastic surgeon would advise you of, such as keeping the area clean and dry without excess sweating, wearing a supportive soft bra without underwire, and padding the incisions where they contact the bra. The less touching, manipulation, showering, and bra changing the better. The less arm movements overhead the better, and anything that keeps tension off the wounds so that they are neither stretching apart on the outside nor on the inside. 

Your skin will determine this on its own.  Refraining from smoking will also help this.  I recommend an in-office examination as well as a detailed discussion with a surgeon who you are comfortable with.   Finally, make sure your surgeon is a Double-Board Certified Plastic Surgeon certified by the American Board of Plastic Surgery (ABPS), a member of the American Society of Plastic Surgery (ASPS), a member of the American Society of Aesthetic Plastic Surgery (ASAPS), and a fellow of the American College of Surgeons (FACS).Best,Dr. DesaiBeverly Hills Institute for Aesthetic Plastic SurgeryHarvard Educated, Beverly Hills & Miami Beach TrainedDouble Board-Certified Beverly Hills Plastic Surgeon

Dear cal4nia,wounds on T-junction after breast lift/reduction are usually caused by poor blood flow so there is not much that you can do about it. I would suggest you to follow your surgeon's instructions and keep your wound clean and covered. If you have any concerns, please consult your plastic surgeon. Daniel Barrett, MD, MHA, MS 
Certified, American Board of Plastic Surgery  
Member, Am. Society of Plastic Surgery

The best that you can do is to continue to follow your physicians advice at this point in terms of post operative activity. You should avoid any heavy lifting, pushing, or pulling for 4-6 weeks to allow your incisions to heal. You should also avoid wearing an underwire bra. Outside of a balanced diet and avoiding smoking...nothing additional you can do will help the t junction heal any better. In terms of scar position, its still early days but the goal of the surgery is to place the scar in or just above the breast fold so that is totally normal.

and your photos appear to show none of that.  On the assumption you are following your surgeon's recommendations, there is really nothing else that you can do to prevent a wound separation.  And on the outside chance one does develop, with good wound care, it will heal exceptionally well.  Your surgeon is only mentioning a common problem with the anchor incisions... but yours appear good at one week and its unlikely anything adverse will occur.