Two firewalls and internet line

I just recently been introduced to clients who currently have one firewall in place, sonicwall. They do have another internet line ?different ISP” not in use yet, it’s been installed but not configured to serve the LAN yet, they want it purely for VPN connection. I’ve been asked on how to make use of both line in best possible way, either by utilizing the existing firewall or introducing another one and make it purely to serve the proposed VPN, etc. Any idea what’s the possible way to do it knowing my knowledge with the sonicwall is very limited? Any suggestion or direction would be greatly appreciated.

Regards

Jassem