"but experimenting with IPv6 to be prepared for the eventual change."
And I commend that fully.. I dabble with ipv6 myself to keep my hands in it for when might actually use it at work.. Which is truly lagging, I will most likely be retired before ipv6 is fully mainstream to be honest.. I would suggest go take certs tests from HE, you can get a free tshirt when you pass sage level.
I by no means am a dhcpv6 expert, but what dok mentions is going to be where you get started.. The DUID is going to be per machine, and this is normally how a dhcpv6 would give you your IP.. Its not going to give you multiple because you have multiple interfaces in the same network.. Now this could be tied with the IAID I assume to allow you to have each interface get an IP in the same prefix.
You would have to read the rfc's to be sure.. And then again would depend on if pfsense supports that, and if your isp support that..
I don't really see when this would be useful though. Such a setup shouldn't really exist.. Why would you put 2 interfaces from the same machine into the same network? Especially on a ROUTER!! You might do it on some host I guess serving up websites or something on different IPs.. But on a router - no.
Simple solutions to your problem.. Use different isps ;) So each interface would get its own IP in its own prefix.. Use HE for ipv6, you could for sure setup tunnels on each interface.
Even if pfsense supports having each interface get an ipv6 in the prefix, doesn't mean your ISP does.. You could contact them - good luck with that ;)