If only one of the tunnels of site to site vpn is down, is communication possible?

Yes, it is certainly possible, even with 1 Tunnel you could have bi-directional connectivity.

A Site-to-Site VPN connection consists of two VPN tunnels between a customer gateway device and a virtual private gateway or a transit gateway. It is recommended that you configure both tunnels for redundancy. From time to time, AWS also performs routine maintenance on your VPN connection, which might briefly disable one of the two tunnels of your VPN connection. For more information, see Tunnel endpoint replacement notifications.