Yes, you can select to put, say, sum of input 1 and 3 to Left and sum of input 2 and 4 on Right. (Or just about any combination).
Its in the manual, you might consider checking it out, page 24, although I agree it is quite convoluted reading:
http://www.sounddevices.com/download/guides/744t_en.pdfThere are a few disadvantages though. First is that you have very little control over the relative volumes, no mixer as such only on/off. But main disadvantage is if what you are after is the "heavenly sound" of the MR1, the sound is already "spoiled" by the 744. The reason is that it is first AD-converted and this is the signal recorded on disc. Secondly it is DA-converted and output to the line-out. And after that it gets AD-converted again in the MR1. Now, admittedly both AD and DA in the 744 are good stuff.
Hope this helps, not meaning to come across as anything but helping, sometimes internet gets in the way (or perhaps it is writing in a second language).
Gunnar