Contact Kraiburg. They have grades just for ABS and other engineered resins. I worry that your supplier is not molding the substrate, then over molding the part. That will make the control of the two melts very difficult, and increase the scrap produced. Also you will have a hard time finding a viable means to recycle the scrap parts from these types of processes.
The best method is to purchase a press just for this process. Then inject the ABS, and pull the core or rotate the mold halves, then ASAP inject the TPE onto the substrate. The higher the surface energy of the substrate, the better your bond strength will be.
Hope this helps, Rick.