I just purchased an R7 260X. When using the released Windows 10 driver, the secondary monitor does not work with MixVibes Cross video output. Slideshows, and continuously crashes the video driver over and over.
I have removed the windows 10 driver and latest version of catalyst and installed the Windows 8 driver that came on a CD from the manufacturer which so far works perfectly. I have had to disable the automatic driver updates in Windows 10 to prevent the unusable driver from being reinstalled automatically. (very frustrating, but that's a windows problem not yours)
I have submitted a bug report through the feedback system already with my system information and driver versions.
I would appreciate it if someone could take a look at this issue and fix it for future driver releases. The driver makes the card unusable with this software, which is one of the primary reasons I chose this card, for the multiple monitor support. I would imagine you are going to have some grumpy customers, who are less technical with their systems, having the same problem when windows automatically installs this driver when they upgrade.
Note the circa 2008 Nvidia GT 9800 card I just replaced has working windows 10 drivers that does not suffer from this issue, so I know that Mixvibes is not the cause of the issue..