You can get images as numpy arrays, so you can very easily combine them in any way you want using numpy operations.
Just a few illustrative examples (I haven’t tested these expressions, so syntax may not be fully correct, but something similar should work):
Get minimum of two images: imageCombined = numpy.minimum(imageA,imageB)
Get image A voxels if it is above a certain intensity threshold, otherwise return the mean of image A and B: imageCombined = numpy.where(imageA>1500, imageA, (imageA+imageB)/2.0)