depends...
do you have room on the plenum to add another duct?
does your unit have the capacity to supply another duct?

one ton of ac is 400 cfm. (cubic feet of air per minute)
each duct size moves a specific amount (cfm)of air.

flex and hard pipe ducting makes a difference
and length of the run effects the amount of air moved.

call an hvac company and get their input.
otherwise you may make more of an issue than you have
to start with.

best of luck.