The encrypted volume of the Nitrokey Storage can be unlocked by entering the User PIN which is the same for the smart card. Your suggestion would implicate that encrypted volumes would need to be unlocked with a PIN different from the User PIN. Otherwise the PIN could be easily verified (and a false PIN could be unmasked) by accessing the smart card. This additional PIN would sum up to five PINs/password: User PIN, Admin PIN, Firmware Password, PIN for encrypted volume 1, PIN for encrypted volume 2. I think from a usability perspective this is way too complicated for most users.
In addition the available storage capacity would need to be considered. For instance, do you want to split the available storage between both encrypted volumes? This would result in 50% less capacity, for users who don’t want to use the 2nd encrypted volume.