Which is the easiest way to create a composite false color from a image stack?

[GM]

In a stack, to perform registration, I've two RGB images of the same subject in different spectral bands . After registration I would like to create a false color image directly from the stack using two channels from the first RGB slice and one from the second. Usually I perform this procedure: Make stack image->Registration->Stack to image->(Color->Split Channel)->(Color->Channel Merge). Is there already a faster way to do that directly from the stack or I should think to write something on my own?Thanks a lot.