Working both sides at the same time

Hi everyone. I'm knitting my first sweater, and can't work out the "work both sides at once" bit. First, though, I was wondering if anyone knew a good source for a video or diagram, so I can understand the explanations I'm finding. Here's the part of the pattern I'm unsure of:

"...join a 2nd ball of yarn and bind off center
24 sts, work to end. Working both
sides at once, bind off 4 sts from each
neck edge twice..."

Any help would be great. Thanks, everyone!