Looks like some confusion about what Base6to4Interface is used for: Base6to4Interface is only used for 2002::/48 addressing (aka 6to4), so you could have this:
prefix ::/64
{
Base6to4Interface eth0;
};
but only if you wanted to announce one of the /64's of your 2002:4658:b266::/48 range for an interface. Basically the option reads the IPv4 address on the interface specified and uses it as part of the IPv6 address. It doesn't look like you have sit0 configured so you don't have an active 6to4 configuration - so remove all Base6to4Interface entries from radvd's config file.
Your sixbone tunnel is referred to as a 6in4 tunnel, and he.net will assign two or three prefixes to it (/64 link, /64 allocation + optional /48 allocation)
The /64 link range should _only_ be assigned to the sixbone interface. The /64 allocation can then be assigned to _one_ other interface of your choosing. If you need to allocate to more interfaces than this you can split up your /48 and use that. Notice that you can't have the same range assigned to more than once interface at a time.
So it looks like:
link range: 2001:470:4:35::2/64
allocated /64: 2001:470:5:35::/64 (this you don't have configured anywhere yet)
allocated /48: 2001:470:d848::/48 (it looks like you assigned the entire /48 to eth1)
A minimal of what you want is something like:
sixbone: 2001:470:4:35::2/64
eth0: 2001:470:5:35::/64 (notice 4->5)
eth1: 2001:470:d848::/64 (notice /48->/64)
An alternative is not to use your allocated /64 and configure something like:
sixbone: 2001:470:4:35::2/64
eth0: 2001:470:d848:1234:/64
eth1: 2001:470:d848:5678:/64
because you've used two or your 65536 ranges from your /48 for eth0 and eth1, you can pick the '1234', and '5678' parts to be whatever four hexadecimal digits you want.
so an example radvd for those ideas would be:
interface eth0
{
prefix 2001:470:5:35::/64
{
};
}
interface eth1
{
prefix 2001:470:d848::/64
{
};
}
or
interface eth0
{
prefix 2001:470:d848:1234:/64
{
};
}
interface eth1
{
prefix 2001:470:d848:5678:/64
{
};
}
Obviously you need to update your routing tables to match too.
Hope that helps,
Cheers