You should have some papers with the AHU that tell you what the static capibility of your AHU is at 1640 CFM. Typically they would look like .2" SP @ 1700 CFM, or .4" SP @ 1500 CFM, etc. and it is usually in a chart form. The static pressure (SP) listed in these charts is telling you what CFM the AHU is capabile of delivering against the losses in reference to the duct system, duct fittings, filter, grills, etc. The return duct if any counts also.
If there is no reason to split the duct trunk line (get around obsticles or what ever) I would recommend just one main trunkline. Simpler is cheaper. Minimum bends, fittings, etc. , your AHU should be able to handle that
Typically you can size a duct system for .05"SP/100' of duct and be OK (especially for a furnace with cooling coils). Alot of contractor size for
.1"SP/100'. You can probably find a "Ductulator" for sizing duct on the a web page or at your Local Trane dealer.