Sorry Adamski, I was sure that I'd responded to this.
I think it's partially random, but at the same time there'll be an element of 'law of averages' about it. Bear in mind that some leagues have rules against B teams being promoted to within certain divisions of the top tier/parent club. Spain is 1, I think? I'm sure Germany is possibly 2 etc.
I'd imagine, though, that if you gave your B team enough talent, they could probably get promoted fairly quickly, though