Is It Normal for a Tummy Tuck Incision to Be Splitting Open?

I am sorry to hear about the complication you are experiencing.  Superficial separation of the incision line is a known complication after tummy tuck surgery.   Close follow up with your plastic surgeon will be in your best interests; sometimes removal of a exposed suture and/or removal of any unhealthy tissue will expedite healing. Otherwise, wound care regimens will differ from one plastic surgeon to another.  Generally, this will involve application of some type of sterile dressing ( in my practice I use a non-stick dressing covered by a sterile dressing). You should have peace of mind that these types of wound healing problems generally go on to heal over the course of the next several weeks,  without long-term sequelae. Also important to concentrate on a healthy diet,  including good protein source. Occasionally, in the event of a wider scar, scar revision surgery may be helpful down the line.  Best wishes. 

This is not normal. You need to express your concern to your surgeon and if they don't address it, please seek the advice of another surgeon who can. Best of luck.

 Unfortunately this is a complication of the procedure and is referred to as an incisional dehiscence which can have multiple causes including poor nutrition post-op, infection, smoking and increased tension. If you can, you may want to increase your oral protein intake to 60 gm per day with a protein supplement. Look out for infection and protect the incision by keeping slightly flexed at the waist. Other things that can contribute to it are smoking and even second hand smoke, as well as underlying fluid collections. 
Good luck
Dr. Trussler