2 x Fritz Box with 2 different networks on a laptop possible?

I got my fiber optic cable activated today, but since I still have a contract with my old internet connection until December, I would like to use both NRTs. Is that possible?

If I want to register the new one via Fritz.Box, only the old Fritzbox comes

Yes, with the cable box on the Internet.

With the previous one via LAN to the new router.

If the IPs are different, you do not need to change anything.

I wish you success.

Both networks can only be used within the house. With online availability in the new network, there's no way to continue using the previous provider.

These are probably different techniques.