Yes, the Womb Music will work with twins, triplets, etc. also but here is one thing you should keep in mind. Typically twins will be smaller than a single baby at any given point during the pregnancy. So when you are looking for your twins, you'll experience things in a similar way to someone who is not as far along, i.e. - if you are at 16 weeks with twins it may be more like listening for them at 12 weeks for a single baby. But other than that, the device should work the same for you as any mommy. You just have to find the baby and assuming one isn't in front of the other one you should be able to find distinct heartbeats for each.
Keep in mind that the monitor has a very narrow focus so when you find a baby's heartbeat it comes in very clear. But if you move to quickly or angle/move it away from pointing at the baby's heart, it will not be heard. This is actually a good thing because it allows you to focus on just one heartbeat at a time. It's sort of like using a laser pointer vs a floodlight. The monitor is more like a laser pointer that when it is pointed right at the heart, you'll hear it. So as long as the babies are not blocking one another you should be able to find one and then move to another position to find the other one.