@richinaus not an odd question at all. What you want to do is possible and there are success reports on eGPU.io forum. It is a rather advanced implementation. The key is to use a ghost display adapter which connects directly to the eGPU output port so that apps launched through this ghost display have full acceleration. You can then switch these eGPU-accelerated apps to the internal display or the 5K display.
Step 1:
15″ MBP « » TB3 cable « » Thunderbolt 3 eGPU
Check System Information » Graphics/Displays to see the GTX 1080 Ti is showing with Metal support. Run an OpenCL app such as Luxmark to confirm the eGPU is working with your MacBook Pro.
Step 2:
15″ MBP « » TB3 cable « » Thunderbolt 3 eGPU « » Ghost Display adapter
Make sure you can switch apps from the ghost display back to the internal display. Once you accomplish this, proceed to add the 5K Thunderbolt 3 display into the mix.
Step 3:
5K TB3 Display « » 15″ MBP « » TB3 cable « » Thunderbolt 3 eGPU « » HDMI adapter
At this point there should be 3 displays: 5K, MBP internal, and ghost. The ghost display should be set as primary so that all apps are launched through it by default with eGPU acceleration. You would then proceed to switch the app to the 5K display through the use of Spectacle.
P.S. If you read all this and said to yourself this is confusing af and why would anybody go through all these steps to make it work, I feel ya. Apple could and should support eGPU so that it makes life easy for everybody.