Alternatively you could use the blitz3d texture blending features to do this:
Create texture which is all r,g,b=128,128,128, multiply blend this with your first texture. Repeat with second texture. Then add the two resultant textures together using an additive blend mode and save the resultant image.
Create texture which is all r,g,b=128,128,128, multiply blend this with your first texture. Repeat with second texture. Then add the two resultant textures together using an additive blend mode and save the resultant image.