I have a base object, and I also have as a separate object a quad with a texture and a normal map on it. I'd like to apply these to the base mesh.
Using texture painting and stencils is out because (as far as I can tell) you can't be precise with stencils, you need to line it all up by hand, and that won't work for an object with texture and normal map.
Whenever I attempt to use Bake to Selected, I get weird results. Often the bake will partially work, but the texture will also spill over onto any back faces behind the decal on the other side of the base mesh. Depending on arrangement and size and other things I don't know about, sometimes it'll also bleed over to other weird places. I've tried with and without the Cage modifier, and Cage makes the issues far worse. A small but non-zero ray distance and no cage seems to be the best.
Here's an example if you'd like to investigate - http://puu.sh/DEOmu/d24afcb646.blend - I'm using 2.8 and I've simplified down to the minimum to demonstrate it, just one object and one decal with one normal map texture (but it happens with diffuse colour also) - have a look at the back face and the weird distortion.