FlexResponse,
Share common ducting yes, but the aft isolation valve is normally left in the open position for engine starting.
I personally have always found that if the APU has low(er) output, starting a wing engine first has produced better results, due to higher duct pressure, and the fact that a wing engine only drives one hydraulic pump.
Some might disagree of course, however as I spent quite a long time on the 'ole TriStar, where it was hot and many times high, starting a wing engine first was preferred, if the APU was marginal.