See perhaps "IPv6 Multihoming without Network Address Translation":

    Network Address and Port Translation (NAPT) works well for conserving
    global addresses and addressing multihoming requirements because an
    IPv4 NAPT router implements three functions: source address
    selection, next-hop resolution, and (optionally) DNS resolution.  For
    IPv6 hosts, one approach could be the use of IPv6-to-IPv6 Network
    Prefix Translation (NPTv6).  However, NAT and NPTv6 should be
    avoided, if at all possible, to permit transparent end-to-end
    connectivity.  In this document, we analyze the use cases of
    multihoming.  We also describe functional requirements and possible
    solutions for multihoming without the use of NAT in IPv6 for hosts
    and small IPv6 networks that would otherwise be unable to meet
    minimum IPv6-allocation criteria.  We conclude that DHCPv6-based
    solutions are suitable to solve the multihoming issues described in
    this document, but NPTv6 may be required as an intermediate solution.

It is generally discouraged, but recognized that it may be needed. Note that NPTv6 is stateless in its address translating (though not stateless with regards to firewalling and connection tracking).

Also (the long-titled) "Enterprise Multihoming Using Provider-Assigned IPv6 Addresses without Network Prefix Translation: Requirements and Solutions":

* https://datatracker.ietf.org/doc/html/rfc8678