I am trying to find a good set of wheels that have hubs with dual cog capabilties. I would like to get the dual setup for longer distances that involve hills and I want the ease of just flipping my wheel. I am looking to spend around the 600 mark and not much more. If there aren't many wheelsets available I am fine with piecing the wheels together too. What do you all recommend?? thanks in advance

It might be faster to give you a list of hubs that DON'T come with a fixed/fixed option.

my formula is fixed/fixed

Yup, Formula makes 'em. Surly makes a fixed/fixed. Phil Woods makes a fixed/fixed. So does Profile, Shimano, Dia-Compe, Miche, Milwaukee, etc., etc., etc..

I run an IRO fix/fix hub (although I cheat and have a freewheel on one side).

For $600 you can get a dang nice set of wheels built up. You are talking about that price for just the wheels and not a whole bike, right?

You can get just about any reputable name builder to put together a set of custom pieced wheels for that price. For distance riding, I'd look at a mid-profile rim and not a deep section. Deep rims become butt-hammers after about 30 miles on a chip sealed or otherwise rough road. The DT Swiss RR1.1 is my personal favourite, with the Mavic Open Pro falling just behind. They're both reasonably light (single eyelet RR1.1), and both strong. The RR1.1 has a wear indicator (if you run brakes) while the Open Pro does not. The Open Pro, IMO, is louder than the RR1.1... I don't know why, but it sounds like it amplifies road chatter and makes a lot of noise compared to the RR1.1 rim. But it's also about $20 cheaper (each.)