We have several RF550 and RF560 routers at remote sites connecting to an RF820. In the past we had to use the same preshared key. With the RF820 router at this end can each VPN tunnel have its own distinct secret (preshared key)?

If all your VPN tunnels are on public static IP addresses, then you can have a unique preshared key for each individual tunnel. If your VPN tunnels are using dynamic IP's, then all the tunnels must use the same preshared key.