Transparency issues

I am having problems exporting a model with transparency. The model consist of two meshes UVmapped to the same PNG with alpha channel. But I need a mesh to be one-sided and the other two-sided. That's why I need it to be splited in two meshes.

I have exported similar models before with no problems, but this I just cannot get it working. I cannot use two materials, as they would need both to be named as the same image file. Or duplicate the texture and change his name, which is another nonsense. And with only one material, if I mark it as translucent I get the transparency working but also the rest of the model looks like if it had inverted normals (wich is no the case). And if I don't mark the material as translucent, the transparency in the alpha of the texture does not work at all.

I have tryed with parenting the meshes and with all the alpha setings in Blender, but I cannot find what am I doing wrong. Any advice will be very wellcome.