It's possible, yes. As long as the LCD has it's own EDID (identification data). Ideally it should be done with a 5000 series card or newer, and you enable EDID emulation for the GPU port driving the 15kHz Nanao. You set up your system as needed to serve and protect that monitor, as it were, and once that's done just plug the LCD into the other port. If the LCD is 31kHz or higher and has it's own EDID, it simply tells the video driver what it can handle and the driver will comply.
I'd mess around with things a bit to make sure you can't wrong-foot the setup, make sure you can't accidentally trigger 31- or 45kHz modes on the 15kHz Nanao, etc. It shouldn't happen, but with frontends targeting different displays and switching resolution, it might. Make sure to check the "hide modes this monitor cannot display" box in the monitor properties in windows, and if you're on Win 10, you can stop and disable the AMD External Events Utility service. Those might help.