Easy,
The answer is yes. I did this for a year before I got a second rigger. There are two types of clips I've seen for stacking lines. I like the ones that have two release clips on them. One is for attaching to the downrigger wire and the other is for attaching to your fishing line. A small wire snap keeps the whole thing attached to your rigger wire. The setup slides up and down the rigger wire and when you are ready to set your second line you simply attach one of the release clips to the downrigger wire and the other to your line. The other stacking setup has a release clip (for your line) and a metal clip that attaches directly to the downrigger wire. It doesn't slide freely on the downrigger wire. You clip it on, then take it off as you bring the ball up. One tip, I found it best to keep your stacked lines separated by 15-20 feet to avoid tangles.

The big disadvantage to stacking is that you have to bring both lines up to check one. Although this wasn't too much of a problem its nice to be fishing at all times with at least one line down. Also, when someplace like Sekiu the action can be fast and furious and you can barely get one line down, much less two.

JMS