you could run in parallel, in that case at the clients network they would have to put a route to your Network via the new RV120W,it depends on their Internet facing setup. If they have more sophisticated equipment then they might be able to enable the VPN with their existing firewall/Internet router.
I believe you can only configure 10 site to site vpn tunnels on he RV120W, am I right? would that be an issue?
I take it there is not a clash of IP addresses between sites.
perhaps need a bit more information about the equipment at the new client site