You need to “claim” the second server into your account. (The same account that you are already using with your old server.) Synology FAQ - Questions, Answers, and How-To's Q15
Afterwards, you can manage both servers from the same web app (and access their libraries as well).
Which one is currently managed is dependent on this selector:
Side note: do NOT put the same external port number into the Remote Access settings on both servers. If you want to access both servers remotely, you need to create a second port forwarding in your router for the new server, with its own external port number.
