This has always been at the back of mind as to how this works or how it just gets in a complete mess.
I've got two ISP's BT and Virgin. Virgin don't supply IPV6 and doesn't look like they will until the earth gets sucked into a black hole. BT does and I've got this working on the draytek 3910
On the pc this isn't an issue Windows 10 takes IPV4 address as the main on and uses that.
On my mac mini which is running the latest version of osx this causes some issues. Some speedtests see ipv4 and try and use that and since the traffic is being routed by ipv6 get stuck in a mess. I guess it could also mess some web sites up as well.
Solution is to disable it on the operating system.
The thing I was wondering though what happens if you have two different ISP's that offer ipv6 how is routing worked out then ?

I guess no one knows the best way to handle ipv6 from two differant isp's

Quite possibly... add loan-balancing and NAT to that list (and if you mention NPT someone will probably throw tomatoes at you).

I still see no one has grasped the nettle

I resolved this on the mac mini by turning off ipv6. Windows and Linux seem happy enough.

And now the question becomes a valid one. I've dumped virgin after a year long fault and got another BT line in so the settings are the same on the 3910 for both connections however ipv6 is only active on one connection and won't activate on the second line.
Looking round it doesn't seem to be possible to have two ipv6 connections from the same or different isp's although information on this rather rare.