Dior, this sounds like an empty threat - P said the same once during an arguement but when it came to the crunch he was sensible over the custody issue.
No court would give full custody to a working father where there was a perfectly good mother on the scene (and you are!) and he will quickly realise that while half custody sounds like the next best thing, in reality it would no more work than him being a SAHD.
in the end we worked it out ourselves as we are not married, and are both too tight and broke to involve solicitors. But if it was acrimonious I would do.
I only know of one family who went for split custody (one week with Mum, one with dad) and im not sure it works.
Not sure if any of this will be reassuring - Im sure youre very upset just now. (I logged on again to order my brothers birthday presents off amazon, couldnt resist that last peek at MN!)
I hope you can both talk about it with less anger in the cold light of day as it were.