I vote for one system.
If you know how to design, then it should not be a problem with only one system, especially when you mentioned a dual-fuel set up... you have your backup already built-in!
Why two, it's not very green or cost effective? More energy is used up and each system will probably be oversized a bit due to the equipment limited small sizes manufactured.
If your serious, then forget the equipment in the attic idea. Sized the duct/s correctly, never ovesize, and seal everything: ducts, house etc...
and don't forget a good fresh-air system....
Note: the only time I would install two systems is when they need more than 5 tons of cooling.
Normally I would go with one system except he's thinking of using heat pump(s). One (1) five ton (approx. 45,000 Btus) HP won't do him much good. He'll be on back up (electric) heat all the time.