At the present time, TomTom distinguishes between units by the email address you use for each, so as dhn notes, you will want to create a second account with some 'other' email address for the 2nd unit. When you fire up MyDrive, you will want to log in with the email address associated with the unit in question. If you log in with the 'wrong' address vs. the device you have connected, MyDrive Connect will discover this and question what you are about to do, which is fine. It offers you a chance to enter the address associated with the unit that you have connected.
We have LONG asked that TomTom get away from the requirement for a unique email address for each device, using instead the serial number (which MyDrive Connect knows) to manage accounts . Who knows ... perhaps some day soon they will make that available, but after all of these years, we're still waiting.
So for now, do as dhn recommends and set up a 2nd account for the 2nd unit, using whatever email address you like. The only requirement is that it hasn't been used by some other owner already.