- 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.
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Proof of eligibility:
Products — Music Software & Audio Hardware | Steinberg -
Transfer license between dongles:
https://helpcenter.steinberg.de/hc/en-us/articles/115001662364-Transferring-licenses-