Changing from Ultegra 6800 crank to r8000 dual power meter

Yes, FC-6800 and FC-R8000 cranksets are completely compatible (except for chainring to arm interface). You did not specify what your current powermeter/head unit are, but I'd check compatibility with whatever's on R8000 just in case.

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.

Actually, with respect, both above answers may be wrong. The inner chainring on the R8000 is about 0.4mm inboard of the chainring on the 6800 crankset. While the Shimano compatibility diagram is a bit hard to read, it says that technically, current generation (including R8k) front derailleurs can shift current or previous (including 6800) cranksets, but the previous generation FDs are not officially compatible with the new cranksets. To clarify: the issue is that with an R8k crankset, the 6800 FD may not move inwards enough to properly downshift and/or you could get some additional chainrub from the FD position. Shimano made the change to improve the chainline on bikes with longer chainstays and/or disc hubs.

That said, 0.4mm difference is not a lot, and that could be within the limits of adjustment. It would naturally be in Shimano's interest to sell you more stuff, so of course they'd say the parts aren't compatible, right? You may wish to Google this issue more. Some comments here say that there is no practical issue in mixing the relevant 6800 parts in. (Not clear if they've tried it or they just think so, and which parts they've mixed.)

In the worst case, front derailleurs aren't that expensive, and the new FD is definitely a significant improvement (easier to set up and adjust) than the old one, plus the new FDs are officially compatible with the old shifters (as shown on Shimano's chart).

The original question didn't ask, but there's a larger issue: drive-side Shimano power meters are likely to be inaccurate. This is due to the asymmetric and complex design of the drive-side crankarm. At a steady 250W, the guy in the link estimates that the drive side PMs read about 7.9% lower than a comparison pedal power meter. The issue doesn't appear to be something manufacturers could easily correct in software (from a conversation with an engineer on a different forum), and in any case, many manufacturers haven't acknowledged it. I believe the guy in the link said that the non-drive power meters should be accurate. I also think he said that previous generation power meters should be more accurate than current generation ones, but there is still some inaccuracy there as they are slightly asymmetric.

This may not seriously affect your training. However, it is a genuine issue, even though it doesn't appear to be widely known. If you must go dual sided, I wouldn't rely too much on the left-right balance metrics you get, and you may want to keep in mind that your measured total power is a bit low.