Can the secondary number be somehow traced? Can one associate the secondary number with the primary number of the device?

The app protects the privacy of your primary number:

The recipient of the call sees an incoming call from the secondary number and not your private number.
When dialing the secondary number, one also does not see the primary number.
The cellular operator on the network where the primary number is located does not see the numbers you dialed via Num2 or the numbers that called Num2; instead, your secondary number appears as the caller or the called number.

However, note that:

If you have voicemail on a primary number, incoming calls to the application will be forwarded to this voicemail in case of no answer, phone busy or rejecting a call. There is an option to set up an app-specific voicemail.
Given a judge’s order, Cellact Communications is committed to providing full details of the calls made via the Num2 app.
If your secondary number is tagged with registration software such as True Caller, the secondary number can be assigned to the owner of the number according to the name of the registrant in the phone book.