I've been closely reading your question:  Have you tried using the Blender node?

Attach one texture node to the Blender input one and another one to Blender input two then use the slider to blend the two?  Depending on your dial setting, you should see both textures...You can also change the color of each input for more effects.

Connect up the Blender node to the Poser Surface's Diffuse_Color channel, perhaps the bump channel too.  The Blender and texture nodes are some of the most powerful nodes in the mat room especially for beginners.  Jan