Thanks, Randolph, for checking that. While I haven't worked on an
Accord it a bit, I really should get an updated manual one day.. This
fact does change the diagnostics some.

On the common path, there must be a different non-shared high/low beam
path, because if they were all the same all lights would go on: one
would not have low or high beams.
On an older car I had, the low and high beams would turn on when you
pull back on the high beam switch - that's why they wired the relay in
series. It most likely isn't really "series", but they wired it into an
OR situation: switch low beams on OR pull back on the high beam flasher
turns on low beams.

See if high and low beam relays are the same type.
If so, try to swap them and see if the problem moves: if now your low
beams work and yet your high beams do not, clearly the relay must be
bad.

If the relays are different, see if you see a change from 0V to 12V on
one leg of the low beam relay when you turn the lights on. You may need
to probe around. Onother leg probably always has12V on it (depending on
how they switched it).

Report back here with your findings -- now that we have someone that
has an updated schematic, you should be able to solve it.