Well, first of all, be careful where you buy, some places will charge premium prices, and send nasty leather out. Next, with any side, there will be a bit not worth using, such as the belly. The belly is usually too stretchy and spongy for quality work, yet places will charge you more to cut it off. So, on to your question... Double shoulders will usually be cut very irregular, so you won't get a whole lot for bigger projects. What you might want to look for is known as a bend. That is cut from the neck to the haunch, and is mostly clean back and side, the back being considered the best part.
Where are you located, and who are you looking to buy from?