Changing from Ultegra 6800 crank to r8000 dual power meter
This FC-6800 and the FC-R8000 are parts based on the same design standard, Shimano Hollowtech II. Here you can find all the compatible pieces : https://bike.shimano.com/en-EU/technologies/component/details/hollowtech-2.html - So it will be compatible.

The only issue you can have if is your new powermeter have an external part on the arm crank, like the battery of the Stages powermeter. This part can interfere with you bike frame. So be sure you have enough space between the crank arm and your frame. This dimension is not part of the Hollowtech II design standard.