If you're going to paralell, I would strongly suggest separate conduits from an "engineering" standpoint. The reason being if a fault occurs in the single conduit, now all the feeders burn up, and the building is left completely without power.
Dealt with this a little over a year ago... paralell feed, 3-phase, 500 MCM in two conduits. One of the feeders burnt up, and tripped the CB on the xfmr. POCO came out and clipped the faulty feeders, and re-energized the remaining feeders, which keep the building operational until we had time to pull the "bad" ones out. Remember that sticker shock from that one too... 500' of 500 MCM copper = $5,000... and the price couldn't be guaranteed for more than a few hours and had to be paid in green cash.
Joe
