If you are a party of 11, you have to call on the phone. I believe the website says for parties of 8 or larger you have to call (or maybe larger than 8).
Having 2 reservation #s is not related at all. If you want to book using the 180+10 on-site advantage, just use either of the reservation #s, it doesn't matter. If you don't need the +10 advantage part, you don't need to give a reservation # at all.
edited: I think I understand better about the 2 reservations - you could do that, book an ADR for 5 and an ADR for 6 -- but you might not get the same dining time, you might not be seated together, etc. If you are a party of 11, you should just book it as a party of 11.