Freewheels and cogs are two different things.
You can use two freewheels, one on each side of the wheel and coast all the time. Or you can use two cogs, one on each side of the wheel and ride fixed all the time.
Or you can use a freewheel on one side and fixed-gear cog on the other, any number of teeth you want, in any combination. The threads for freewheels and cogs are the same. there is no fixed/freewheel side. The wheel doesn't care.
The only thing to watch out for is that some freewheels can't be removed once they are installed without diassembly. You have to punch out a part of the freewheel, disassemble it and remove it with a vice and pliers.