Sounds like a really awesome idea, I’m interested as well. I remember seeing people mold silicone around ABS parts, but there were mechanical overlaps holding the parts together, not just adhesion
TPE (thermoplastic elastomer) is a broad name for many elastomeric plastics. There are several different chemistrys commonly used to make TPEs. I have no idea what the chemistry of Ninjaflex or the other common FDM elastomer materials because I haven’t had the time to look into it.
Generally speaking there are not a lot of adhesives that work will with TPEs, especially rigid adhesives. Solvent bonding may be more viable though since it forms joints that result in inter-molecular forces holding the materials together.
@Ben_Malcheski , one of the most common tpe’s in industry is santoprene. Santoprene can be glued to almost anything with cyanoacrylate or super glue. But I agree with @matthew_bennett , I think the original thought was dualstrusion.