Latitude 610 with Dual Batteries

My school bought 90 off lease 610's with dual batteries.  The secondary batteries are new but the primary batteries are used but we are lucky to get 3 hours of usage.  This doesn't seem right so trying to find some answers.  We thnk the problem is with the primary batteries so we will replace those with new ones but is there a way to get the 610's to use one battery at a time then roll to the second battery.  I would think that we should easily get 6 hours of run time with dual batteries.  Any thoughts on this on how to extend the computers in a mobile lab.  Thanks