As of October, 1990, there was no spooler that could intelligently divert
output to different printers, based on the number of jobs in each queue for
each printer (nor even simply divide all output in half and send each
half to a different printer). Our databases don't show a product that can
do this, nor can the AppleShare Print Server do it. All of the products
listed have a one-to-one spooling capability, and the user has to change
the spooled device manually.
Depending on your software and user configuration, you may want to try
splitting the users into two groups. Instead of having everyone log on
with the same username, make two usernames and have each print to a
different printer. You can put each username into a single group and set
the access privileges at the group level to keep everything the same.