I just returned from a 2 week trip. I brought the X100 and the R-D1 with Canon 25/3.5, Leica 28/2.8 Elmarit-M Asph, Nikkor 35/1.8, Minolta 40/2 M-Rokkor, CV 50/2.5 Skopar and Konica 90/2.8 M-Hexanon.
On any given outing, I never went out with both cameras. I mostly used the X100 for compactness and light weight; at night it was nearly always the X100. Whenever I went out with the R-D1, I took only 2 lenses with me. The most frequently used were (in descending order) the 28/2.8, 25/3.5, 50/2.5 and 40/2; I never took the 35/1.8 or the 90/2.8 out of the bag.
My advice: If you plan to carry both cameras at once, put your 50/2.5 on the R-D1.
Ari