Scenario: Site#A and Site#B have established a site-to-site VPN tunnel.
How to forward traffic to Site#B from a client connected to VPN tunnel on Site#A.
The VPN client can be L2TP/SSL VPN/ IPSec VPN.
The client receives an IP address which assigned by Site#A router.
In this scenario, VPN client gets the IP 10.10.10.1 after connecting via L2TP VPN tunnel to Site#A.
Add a policy route on both sites to forward the traffic.
(1) On Site#A (Rule for traffic to Site#B)
(2) On Site#B (Rule for traffic back to Client on Site#A)
The L2TP client should now be able to communicate with the other site.