Mechanics of Materials would suggest I need to take that moment (9420 in-lb) and divide it by the horizontal distance between the 2 nails (say 2" if each nail is 3/4" from the top/bottom edge). Then i get a tension/compression load of 4710 lb, which i need to divide by my 100 lb nail strength, deriving 47 nails I need along the top and bottom of the splice. If they are 2" apart, that is basically an 8 foot splice with nearly 100 nails. Or I have a 6 ft splice with nails 1.5" apart, staggered a little to avoid splitting.
Doesn't the moment arm between the outer nails count for something? i.e. what if i made an 8 ft splice, but just had (2) nails at each end, and (2) in the middle, (6) total. Having done that in practice, it seems like it would hold far more load than the other method would suggest. I'm obviously missing something.