keeping dash with stand alone ECU

what are the options to keep the dash functional on a newer vehicle when adding a stand alone?

I've been HP tuners reflash all my life but I'm looking at stand alone units and they look so much simpler, but how do you keep the dash working on new cars? or how do you keep the automatic transmission working when you have a PCM and not a ECU/TCU? I'm assuming you run both but seems like it will just throw codes and be a mess.

Really depends on the application, and which standalone. Some brands have CAN data for specific vehicles, some standalones support TCM's or a few can control transmissions directly. Most can interface with standalone TCM's like HTG.

You can do parallel ecu installs, but there are often compromises to make that work.

As the answer often is, "it depends"

What vehicle(s) are you currently flashing?