The following is a explanation on how to manually join 2 uvs.
A simple way to combine 2 uvs [use your favourite app - paint/photoshop..]
is to copy paste the 2nd texture along the x-axis on a enlarged canvas. When you are done with the texture itself
[use your favourite 3d app - blender/max/maya]
you have to set it as the material for your model, you will see that the mesh uvs are stretched on the x-axis.At the end all you have to do is to scale the uvs along the x-axis -0.5 and you have just to make sure that it alligns correctly on the texture space.
Make sure to save your edited models for later export.
There is no magic and to avoid quality loss its best to scale the canvas up and not to shrink the texture!
http://j0linar.blogspot.co.at/
PC SPECS: Windows 8 Pro 64-bit, Intel I7-3630QM 4.8GHz CPU, Nvidia GTX 675M - 2048MB GPU, 16GB DDR3 RAM