Thank you for your reply.
There are no IPv6 clients in the server's network. The server is located in a datacenter which doesn't provide native IPv6 connection.
That's why I want to test out he.net and SixXS tunnels on this server.
I already set up a SixXS tunnel and a he.net tunnel but I can't get them working at the same time.
The SixXS tunnel works and the Server is reachable over IPv6.
If i add the he.net tunnel with
ifconfig sit0 up
ifconfig sit0 inet6 tunnel ::*Server IPv4 Address*
ifconfig sit1 up
ifconfig sit1 inet6 add *Client IPv6 Address*
the server is reachable via the SixxS IPv6 Address but not via the he.net IPv6 Address.