Does anybody know if it would work to not enter your FF# when you book, and send in the boarding receits later, some to CO and some to NW?

I certaily would like to split an upcoming trip after Nov. 15th -- NW ticket, CO metal across the Pacific, and NW metal to the next stop. It would get me double miles on my NW account for the purely overseas leg. I have a few miles in my NW account from pre-"cooperation" and would like to get it up to a reward level.