The issue is the merging of two different kinds of silicone. Silicone in itself doesn’t like other silicones and the harder ones tend to “melt” the softer ones (in reality the polymer chains interact and cause migration, which deforms the softer object as it will have higher plasticity).
You could make a hard silicone dildo with soft silicone balls but most likely you’d end up with the ballsack tearing off in a play-doh consistency mess within a few weeks of manufacturing.





Sounds like a challenging wank, no?