I'm editing the culture file to create a custom culture and am creating my own name list. What i'm wondering is where you have two names joined by an _ will both names have a chance of showing up?
So for instance if I had "Robert_Bob" would both Robert and Bob have an equal chance of being used?
I haven't tested it, but think that it would generate someone with a double name called Robert Bob. Not just Robert, nor Bob.
No, what names with underscores do is connect different language version of the same name. So, the English culture has Robert_Robert, while Italian culture has Roberto_Robert & Occitan has Robèrt_Robert. This lets the game know that Robert and Roberto are really the same name. So King Robert III can be followed be King Roberto IV.
Putting 2 names separated by a space within quotes is how you make a double name like "Sarah Jane".
These are both actually discussed right there in the link Salciano put up.