I already have Win 10 installed on my NUC.
Can I create a second partition from Windows and install El Capitan there, following the instructions in this thread? I see the procedure for the other way around in this thread...
Yes it does. That is one of the models that the guide in the first post of this thread covers.
Here, however, the user tomchak http://www.tonymacx86.com/general-help/167491-nuc5i7ryh-wrong-system-profile-2.html asks the same question myself and being told not to buy because it is not working.
OS X is not well behaved with disks partitioned by other than OS X Disk Utility, but there are things you can check/do.
See this thread: http://www.tonymacx86.com/multi-booting/133940-mavericks-windows-8-same-drive-without-erasing.html
Two options: there's a setting in the BIOS to "always boot USB devices first" that you can use, otherwise the F10 hotkey to select a boot device and then select your USB drive.