This is what Avid's Knowledge Base has to say:
https://avid.secure.force.com/pkb/ar...ate-Crossgrade
Q: If I purchase a Pro Tools Crossgrade, what will happen to my license?
A: You will receive a subscription license (Pro Tools Subscription or Pro Tools Ultimate Subscription Bundle) that will give you access to the latest Pro Tools version available. This will require you to surrender a perpetual license.
Note: Once you surrendered your Perpetual License it will convert your license into a subscription. Once it's done it will be irreversible.
If I understand that correctly, then you surrender your perpetual license at the moment you crossgrade to a subscription license. AVID will let you have only one license.