Black Diamond makes a "pole link converter" to turn two trekking poles in to a bigger tent pole. Handy, but it does leave the points exposed on both ends, so you might want to carry rubber tips so you don't poke a hole in your tent. $5, 35g, but you'll probably have to order it and pay shipping. I've never seen these in stores.
Or you can use the Ron Bell method with a few feet of guy line. I can't find a link to that right now. Roughly, you overlap the two poles, points down, then put a small loop over the point of the upper pole, thread the line through the strap of the lower pole, tie it off somehow (hmm, need to try this), then wrap the remaining line around the two poles.