Exactly this; change one of the Xpenology instances MAC address - https://www.mattharmon.co.uk/2016/06/how-to-generate-a-valid-mac-and-serial-pair-for-xpenology/ (hopefully this link doesn't break forum rules).
I thought that I already had done this as both microservers have been running fine, together, on the same network for the past 3.5 years.