Newbie question here, do both sides of ALL flip flop hubs have the same threading? Can I remove the fixed cog and install freewheel so I will have freewheel on both sides Or vice versa? or is one side dedicated to fixed cog only and freewheel one side? Reason I ask is because fixed cog has a lock ring (that threads opposite direction) and a freewheel doesn’t.

No.
The fixed side has a slightly smaller diameter section for about half the length for the lockring. This makes a freewheel possible, but weaker with only half the threads holding the body to your hub.

so I would need different hubs specific for freewheel cog only?

Huh?
There are track hubs, with fixed on one side only.
There are fixed/fixed hubs
There are fixed/free hubs
there are even free/free hubs, but they are rarer. That's because there are easier ways to have multiple freewheeling gears.
You said you have a fixed/free wheel, which means one side is threaded for a freewheel, and the other has the smaller diameter shoulder to accept the lockring, so your fixed cog doesn't spin off from backpedaling.

If the number of threads is sufficient for a track cog, why wouldn’t it suffice for a freewheel. Very powerful track sprinters race track bikes with fixed track cogs without any problem. So, the correct answer is yes, it’s perfectly fine to use a freewheel on the fixed threaded side of a hub.

tejano, thanks for the clarification. I just want to make sure the freewheel will not get damage installing it on a fixed threaded hub.

Rest assured that it won’t be a problem. The freewheel and fixed cog threading is exactly the same.

thank you!

Unless you want them different.. freewheel-fixed combinations are made..