Right now I had to use tobiaswallin.se for IPv4 and ipv6.tobiaswallin.se for IPv6, since I didn't manage to make tobiaswallin.se to point to both IPv6 and IPv4. I'm not sure I understand your question, but the dynamic update of both the IPv4 and IPv6 adress is done by my router which is a FritzBox 7490. The router will first make an update for the IPv4 address, and then an update for the IPv6 address. Maybe the second update will erase the first?
As a separate note, the ZoneEdit web interface won't allow a dynamic IPv6 address to be updated manually on "DNS Settings/Streamlined Editor" page. When pressing the "next" button the data entered is refused.