I don't run a herbie, but I do have dual 2" drains with about 1200 GPH running through them. In my case the 2" is overkill and I wish I would have went with 1-1/2". It would also help keep your plumbing a little more tidy, those 2" unions are huge. This chart may also help:
http://flexpvc.com/WaterFlowBasedOnPipeSize.shtml