ryan is pretty much on the money.
however kor'axis, the Y inlet would infact "bottleneck" the air flow to both throttle bodies, thus making the whole twin throttle body idea useless...
but don has done some calculations and if you look at his last post above, you will see that the area of 1 rb25 throttle body is 60mm in diameter this means it has an area of roughly 28 square centimeters, and since he has two throttle bodies - that will total to 56 sq(uare) centimeters.
now, in order to overcome this Y bottleneck, the afm must flow at least equal to or more area than the two throttle bodies combined - thus making the q45 afm the best option. the rb25 afm is too small by 12 sq cm, and the z32 afm is just under by 6 sq cm.
however! don has also mentioned that the last time he ran the single rb25 afm he didn't max it out - thus making the z32 afm a possible option as well.
to put it simply, don will theoretically have twice as much air flowing.
rb25 afm "may" restrict the 2 throttle bodies, the only way to know is to put it on and see if it maxes out.
z32 afm is theoretically just under the area don needs, but will see in future if he decides to use one - and if it maxes out.
q45 afm is theoretically the most appropiate for the job, capable of flowing much more than what don requires - will see how he goes.
keep us posted don!
on another interesting note:
Also in an R31, and also NA - but this runs an RB30E