Your assumed load will be on the margin of single vs double portage requirement.  Both members need not double portage, only one.  If speed is your goal, have the single portager unload and load the canoe, while the other doubles.  When you land, the latter takes off immediately with a pack.  The former unloads and takes the canoe across, then loads the gear the already sitting there.  Since many portages are 20 minutes or less round trip, I think you will find the double portager returning with the 2nd pack as things are ready to shove off.  

-YB