When I first picked up both the 868(DMR-6X2) and 878 I saw people write that you can upload an 868 codeplug to an 878. I've never been able to do this directly. But, I have figured out a simple way to synchronize the 868/878 codeplugs.
First off, open either they CPS for the 868 or 878 and edit the settings until you are happy with them, or at least ready to test.
Within the CPS software select the Tool menu and then "export Data Conversion File."
Save this .dcf file to a location you can easily find again.
Open the CPS software for the OTHER radio, if you started editing in the 878 CPS, open the 868 CPS now.
Once the new CPS instance is open, select File -> Open, browse to the spot you saved the dcf file and ensure the File Type drop down is set to "dcf" so you can see it. Once you've found it, go ahead an open it.
Now, the CPS software for the other radio should be open and have the dcf file from the first CPS software loaded. Head back to the File menu and select File -> Save As. Make sure that you have the File Type dropdown in the file picker set to "rdt." We don't want to save another dcf, instead we want to save the data loaded from the dcf to a standard "rdt" file. Save your rdt file where you can find it again.
Close the CPS software.
If you have a preferred Hex Editor, open it now, if not, get one. I prefer HxD, google it.
Fire up your Hex Editor and load the rdt file that we saved in the previous step.
Below is the location we are interested in, note the status bar of HxD shows that we are at offset 11. This is the setting that will produce the Band Error message if we were to try and program the radio with the imported dcf data. The D878UV's rdt file is set to "A" at offset 11, the D868UV(DMR-6X2) is set to 0 at offset 11. Change offset 11 in your rdt file accordingly. As stated, this is an 868 so it is set to 0.
Save the rdt once the value is changed, close your HexEditor and open the CPS software for that rdt file.
Make sure the CPS is loading the rdt we just fixed, if not, find it and load it. Proceed to program your radio, if done correctly you should have identically programmed radios and no "Band Error" message when programming.