OK - well you can do it a couple of ways.
If you're coordinated enough, you could feed your DS on one side while pumping off the other at the same time. You'll be able to express a lot this way because you'll be able to capitalise on the let down you get while feeding him.
Alternatively, pump off one side. If you get as much as you want from there, feed your DS off the other side. If he still wants the second side, just put him on the original side you pumped from. Or, pump off both sides, then start him off on the first side you pumped from. He'll still get what he needs - breasts are warehouses, not factories. While they do have the capability of storing, you will make milk directly from your bloodstream during a feed if there isn't enough in reserve. Cool, huh? 
Other good pumping tips - you'll get the most by expressing first thing in the morning; try and rhythmically compress your breast while pumping, squeezing gently from armpit to nipple to make sure you're evacuating the milk ducts effectively but also allowing them to refill between compressions; express after a warm bath or shower - again you'll get more.
Also - remember you can use cow milk (if your DS doesn't have any CMPI or anything) in food like porridge, as it's used in such small quantities. It doesn't have to be EBM, although both of mine did seem to prefer the taste of EBM in food over cow, not surprisingly. Prepare yourself for the smell, though. My EBM rice pudding used to smell like puke! 
(Both my boys inhaled it though... 
)