This so, and this is the same if you use IPv4. IPv6 does not bring any regression here; sadly, no progress either. If you have a server that listens to requests though, such as an HTTP server, I don't see how this setup would be grossly inadequate for the purpose.

I would experiment with advertising two default routes, one with a significantly higher metric than the other. Most / all outgoing traffic would go through one link then. If you want to optimally load both uplinks, you likely need a more intelligent (reverse) load balancer.

> If you have a server that listens to requests though, such as an HTTP server, I don't see how this setup would be grossly inadequate for the purpose.

That's the problem. It sounds like it would work if you do this. The documentation suggests multi homing like this would work. When your server gets a request, it sends back the response from the address it received on... but the problem is what router it sends to; when it sends to the correct router, everything is good, when it sends to the wrong router, that router's ISP should drop the packets, because they come from a prefix they don't know about.

> I would experiment with advertising two default routes, one with a significantly higher metric than the other.

Sounds like it would work, but as far as I've found, the priority metric only works if the prefixes are in the same advertisement. If each router advertises its own prefix, the actual metric used is most recent advertisement wins as default route.