François Tarlier has released yet another Pixel Bender filter; he says Pixel Bender : ft-CombineRGBA for After Effects is the last one this week:
"This really simple shader let you combine 2 inputs channel into a new one. There is already something like that in AE, but I believe it is only working in 8 or 16 bits. This one is working in 32 bits (thx pixel bender). I just did it because sometimes I do need to combine map datas (as normal map & height map) for my shaders, since Pixel Bender is limited to 2 inputs."