If you want something dynamic, then you'd look for a BGP based tunnel. You can only get one of those if you have your own ASN and IPv6 allocation. You'd then configure 2 BGP tunnels and announce over both and sort out the load balancing on your side. Other option is with 2 unique tunnels, a bunch of source address routing policies configured on the router with the 2 tunnels terminating on it.
Hi, thanks for your answer...
For the BGP based tunnel: how can I get an own ASN? How many cost it?
Otherwise, could you explain me the second option, with two unique tunnels? What I need is that some devices are reachable from Internet with their own IPv6-Address. Is it possible (without a BGP tunnel)?