Hello Sam of @samrobley,
This is a topic on polymer miscibility. Not all polymers mix well and sometimes they don’t stick to each other as molecular diffusion cannot take place that well. It is quite a complex topic that I myself don’t fully understand. Having said that, I have successfully printed PC-ABS on Ninjaflex with relatively good fusion between the two. Now you know it sticks. How it behaves in a dual extrusion setting nobody knows until you try it. Another lead I found useful was to look at ultrasonic welding of polymers. Some polymer pairs weld together well and some don’t. Stick to processes mixing or joining polymers using heat and you’ll have your answer.
Here are some ideas from the top of my head if I were you: Start small and print a small cube with the lower half in TPE and top half in ABS. Try it the other way around. Try to mix TPE and ABS within a layer and see what happens. Do all this while observing what happens and be ready to cancel the print if something goes wrong. Worst case scenario is a dirty nozzle so I don’t see any problem with experimenting with this. Gradient properties is currently an interesting topic where you can manipulate the stiffness of a part in different areas. So I’m happy seeing someone else experimenting with this.