I have not attempted a gateway pool yet.
And that doesn't work with a gateway pool anyhow, any attempt to send out traffic from a HE address through the Sixxs router will fail.
The only way to make that work is by performing NAT and there is no provision for that.
You can not add multiple v6 subnets on the same interface either.
The only trick you could theoretically use here is performing network prefix translation.
So if you have 2 /64 subnets, one from each ISP you can then translate this network when traffic leaves the network.
I do this by assigning a ULA range on the LAN, then create 2 mapping, 1 for each WAN with each network prefix. This so that traffic leaving either interface is using the netblock from the correct ISP. Not much different from ipv4 nat, but subtly different.