I'll try and explain better.
I currently have a IPv4 MPLS that connects the two sites; the second site pulls Internet traffic through my location; however, that is changing today...they will be getting a 20mbps Internet connection and will be using the MPLS to connect to our servers. I would like to set up a IPv6 connection between the two of them, so I assume that means I will need a tunnel between the two. Although the more I think about it, I think I would be smart to also request a new /64 for that location as well (so Internet bound IPv6 traffic can leave at 20mbps instead of 400kbps.
My current IPv6 router is outside the firewall, so any traffic intended from it to the second location would be sent over the MPLS.
ASA's do not support tunnels.
It'd probably be a good idea to just get a second tunnel and /64 or /48 for the second site so you can use its new inet connection to connect to the IPv6 internet.
But there's nothing stopping you from routing any IPv6 networks you wish between the two sites via the MPLS connection for site to site traffic. It would make sense if the MPLS connection is faster than both sites' internet connections. Also, it'd be a bit more privacy for your inter-site IPv6 traffic.
You can simply set up a 6in4 tunnel through the MPLS connection and route your IPv6 networks across that.
Or, if your MPLS routers support IPv6, couldn't you set IPv6 addresses on your MPLS interfaces and route it that way?