I noticed that there's not much info(except the online docs of course) on double-sided materials available in Modo, so here's the YAMQT(Yet Another Modo Quick Tip) about it. Double sided materials are especially useful for all sorts of thin objects such as cards, tubes...you got the idea, if there's no point to spend additional time on creating/mapping thin geometry(distant shot perhaps?), double-sided material is there to help. In this particular example I'm gonna look at the tube with some markings on the outside and some bump map visible on the inner side.
If you want to switch sides, you can either do this with the gradients values, or via "f" key, which flips the polys - assuming you haven't remapped that key to other command....
As you can see in this example, MODO's viewport is not displaying this effect, so it's better to check the Preview renderer to get the proper feedback.
Hope you'll find it useful! Scene available here -> http://bit.ly/1d2TFuo