Can you apply more than one decal to a Union?

I've seen a tutorial on how to make a union have decals on all sides, however, it requires PhotoShop / a 3D Model Editor, according to the writer. Is there a way to apply more than one decal to a union via scripting or otherwise, without any external software interference?

Technically, you can place more than one decal on a union just fine, however, only the most recently placed one shows up. My question is if you can make multiple show up, because I'm using the decal as a custom texture of sorts. (I need at the very least three to show up to have the union covered on all view-able sides.)

P.S.: I'm not asking you to find a way if there is none. I just want to know if it's possible; If it isn't, I can keep the parts separate and sacrifice a little bit of visual flow. However, if It is possible, I'd like to know how to do it to keep the texture looking semi-natural.