Splitting /64 over Multiple Gateway Interfaces in rtadvd

I'm using rtadvd(8) for stateless configuration of machines behind a
dual-legged OpenBSD router. I've been allocated a /64 from he.net and have
everything working fine when using a single-leg configuration of the
router...

fxp0:\
        :raflags#0:addrs#1:addr="2001:470:1F01:x::":prefixlen#64:

fxp0 has an ipv6 address if 2001:470:1F01:x::1

However, when I try and split this network up over seperate interfaces, I've
tried leaving the /64 undivided and numberd tx0 (the second interface) as
2001:470:1F01:x::2 and changed "fxp0" in my /etc/rtadvd.conf to "default"

This doesn't seem to work; I can't get a stateless address assigned.

Will I need to split my /64 into two halves? If so, can anyone recommend
pointers to learning how to do that (I'm not sure if /64 == 2 /65's, etc).
I could check my copy of TCP/IP illustrated, alas it is in storage and
tucked away unreachable :-\

Thank you for any insight.

- Eric