Many different Steinberg (and other) licenses can co-exist on the same USB dongle, so Cubase and Nuendo licenses should co-exist fine as well. (I have 6-7 different Steinberg licenses on my dongle.)
Note: not relevant in your case, but if you would do a Cubase->Nuendo crossgrade, it would replace the Cubase license. But since you will do a “competitive crossgrade” from Pro Tools it will not touch your Cubase license.
I have both Cubase Pro and Nuendo on the same usb elicenser with no issues. The Nuendo license was transferred from the usb elicenser it came provided with to my already registered Cubase one. I regularly move my Cubase license between both usb elicensers with no problems (other than the fact a USB dongle is a complete PITA).
Thanks for this information, this is what I needed to know. I want to be able to put it on my existing dongle, and then purchase a new one and move the license to the new one when it arrives in a few weeks time.