Wow, 500 rhizomes is a good chunk and enough to take up about a 1/2 acre depending on how you are going to set up your trellis system (You can pack in between 1000-1300 rhizomes/acre). I would go ahead and dig your self the same number of holes that you have rhizomes and the little extra work you put in now will alleviate the headache you will have in the future trying to split 2 rhizomes out of 1 hole next year. Trying to split up the root systems next year seems a little dangerous in that you risk damaging them pulling them out of the ground and replanting them somewhere else. If you are doing this as a $$ venture (I am assuming this is so since you are planting so many rhizomes) your primary concern really the first year is to get your plants established and off on a good foot, which basically means taking care of that root system. Your hops shouldn't come into full production until year 3 anyway, but if you mess with digging stuff up next year you may come to regret it.
Another consideration for planting in separate holes (3 ft. spacing) is to reduce the amount of shading. Once your growth really gets going (not until year 2) you are going to end up training between 3-6 bines per plant in future years, and then trimming other bines that come out. With 2 plants in the same area you may not be able to figure out what bines are coming from what plants in future years and will not be able to appropriately cull the bines you don't want growing.
At the scale you are planting you will be a small scale production hops farmer.