Sorry, I couldn’t delete my post in modelign section but I realised this was in wrong section before, so I’m reposting here.
"Ok this one is REALLY wierd…
So I had two objects that I joined together. I had the same surface applied to both. I then went to bake full render, and one of them bakes to uv map and other comes out black. Both are part of the same object as I said. This makes no sense to me at all.
Help? :S"
I have a similar problem trying to get to render a multifaced object where all faces use different parts the same texture. One face renders god, all the rest show as blank.
Does anyone know what the mistake could be?
Marco 
You are not suppose to put curse words in your posts or titles.
You can edit your original post by clicking the Go Advanced button any change the title of any post you have already submitted.
Did both of your UV maps for the objects have the same name?
Well thing is they are both supposed to use one uv map and are on object. anyway, problem fixed on original thread i put in wrong section (funny that :D)
go down to the Objects and Links tab of mesh(s) and remove any thing that isnt the UVmap, or remoe a second uvmap if its there.
Also, about curse words: damnit is a combonation of damn (a verb like ‘curse’) and it… curse it. Its no biggie. And if you mean the use of God in title, thats silly as it assumes everyone believes.
Please keep it clean next time. 
pg