If you crossgrade to Nuendo from Cubase, you will still be able to use Cubase as is, but you won’t be able to upgrade it (Cubase) in future, unless you purchase another license since the original product has been crossgraded.
If you get Nuendo, there is no reason to want for Cubase because it (Nuendo) does everything Cubase does and more, albeit it differently in at least one area, i.e., VCA automation.
I was a Cubase user, and even if I was given a perpetual license to run alongside Nuendo somehow (I can only think a subscription model, which I abhor the thought of) I seriously doubt I would use it, unless there was some exclusive plug-in or some new feature(s) I wanted to test.
I have Nuendo 13, and can’t wait to uninstall 12 so that preferences aren’t copied if I happen to need it (V12) for some reason.