Are any Flip Flop hubs made with identically-sized threads on both sides? As I understand such hubs, smaller threads generally exist on one side to accept a smaller freewheel. Do any hubs exist with the same size threads on both sides so either side can accept the same freewheel?
No. Flip flop hubs by definition have a standard size freewheel size on one side, and a flip-flop size on the other side.
But, there are a few left side drive/right side drive hubs out there that have flip-flop (smaller) size on both sides or just for the left side. And there are left side drive hubs that have the 'standard' size on just the left side - or both sides.
I believe that DK makes/made a hub that was flip flop sized on both sides, then had screw on adapters that converted it up to 'standard' size and you could use any size freewheel on either side, as you wanted to.
Don't confuse flip-flop hubs as being left side drive compatible. That is something different.
EDIT: It's still available - The DK Dual Drive hub. Flip flop sized LSD/RSD design with adapters to allow standard freewheel on either side.
what does a flip flop hub do? and whats the point?
Freewheels screw onto a freewheel hub. To make things easier, a standard size was determined for the threads so all freewheels would fit all freewheel hubs. Unfortunately, about the smallest you could make a freewheel that would fit these hubs was 16 (15?) tooth.
Some people wanted smaller/lighter freewheels so they came out with a smaller thread size that would accept these smaller freewheels (13/14 tooth) and then they put the smaller size threads on the same hubs with the standard sized threads. These hubs were called flip-flop hubs because you could pull off your rear wheel, flip it over, flop it back on, and use a different sized freewheel - on the same bike/wheel!
Very nifty when they were invented.
Today, most people use cassette hubs which allow for much smaller gearing than freewheels could ever do and a much higher quality product than freewheels have ever been. Yet, they tend to cost a good deal more, so there are those who prefer the cost advantage and price advantage that a standard freewheel or flip-flop hub can offer.