with a 744 the channels are 1 left, 2 right, 3 left, 4 right. I assume it's the same or similar.
open up one file (left) in wavelab. go to edit > create audio montage.
once you have a montage look to the left. there is a little 1. click on the arrow beside it > add mono track. add the right track. line them up. then you must render the file as stereo to save it. do the same for the sbd. then open them both in the montage to mix the matrix.