You're not allowed to ask for digital files at Signs 101 but here's a link to a couple of threads that should be of help. They're at a different forum so you may have to register to be able to read them.
you just need to layer side one over mirrored side two. if the colors are the same on both sides , substitute in the file different colors for one side so you can easily recognize and select them if adjustments are needed. Send to quickplot and apply a flood coat of white in between, all done on Imagecal clear. You'll be printing reverse then floodcoat or white mask and then forward.