Why you'd want to take up a second one after going through the hell of the first is beyond me Sounds like madness! (Sorry, my PhD has been an uphill struggle. What's that quote? Oh yes, "It was the best of times, it was the worst of times." Sums up my two and a bit years)
Saying that though, some PhD's from some countries aren't recognised in others so if you wanted to emigrate etc. you might have to do another.