Anyone have deployment experiences they can share on Instana being used to complement & uplift an existing Dynatrace (or AppDynamics) deployment... ?

Hi, it depends how you want implement things.
If you are using either solution in a .NET environment , Microsoft only allow one profiler to talk to the .NET framework.
if you plan to use one agent for APM and the other for Infratructure it may work.
When if comes to JAVA it may work if you can separate the agents in the JAVA applications startup script.

when if come to other technologies it must be tested as the agents may interact in different ways with the code.

One thing to remember is that each agent can be runnnig in two modes, APM or Infrastructure. But when running in APM mode also the infrastructure is enabled.
The best option is to disable the agent that you don't use and only run one agent at the time on each host.
That is my experience.