Usually you just start one VPN on your system to get through a firewall. But if you have to go through a second firewall later it's difficult to configure. That's a common situation if you first have to go through a company or country firewall and then enter your home network which is protected by a firewall. As long a you use a Linux system and a system in your target network protected by a firewall offers ssh access there exists an easy way to get it done.
There is a nice tool called sshtunnel available. You start your first VPN and then start sshtunnel. This tool uses the existing VPN and connects via ssh to a system in the second target network and creates a tunnel in the already active VPN connection. Works like a charm.
I created a small script which starts sshtunnel with customizable environment variables.
TARGET=<ssh target host>
TARGET=<dyndns IP of target>
TARGET_PORT=<ssh target port>
TARGET_USER=<ssh target userid>
sshuttle --dns -vNHr $TARGET_USER@$TARGET:$TARGET_PORT -x $TARGET 0/0