If you intend to start riding freewheel and gradually move on to fixed, just get a fixed/free hub.
I don't see the point in fixed/fixed, they both do the same and if you want to change your gear ratio you can just unscrew the lockring and cog and put a bigger/smaller one on.
It is possible to screw a freewheel onto fixed thread, but you'll only have roughly half of the threads engaged. And why would you if there are products available which are designed to do what you want.
I don't get the remark about a cog being worth an extra tenner. If you want to ride fixed, you need a cog. Can't do without. If you want to ride freewheel, you need a freewheel. which also costs money. So riding fixed isn't more expensive and fixed/fixed hubs do not have more resale value than fixed/free. If you want to start riding freewheel and gradually move on to fixed, you need a wheel with a fixed/free hub, a fixed cog + lockring and a freewheel. Fixed cog + lockring go on one side of the hub, freewheel on the other and whenever you want to switch you can just turn the wheel around.
If you intend to start riding freewheel and gradually move on to fixed, just get a fixed/free hub.
I don't see the point in fixed/fixed, they both do the same and if you want to change your gear ratio you can just unscrew the lockring and cog and put a bigger/smaller one on.
It is possible to screw a freewheel onto fixed thread, but you'll only have roughly half of the threads engaged. And why would you if there are products available which are designed to do what you want.
I don't get the remark about a cog being worth an extra tenner. If you want to ride fixed, you need a cog. Can't do without. If you want to ride freewheel, you need a freewheel. which also costs money. So riding fixed isn't more expensive and fixed/fixed hubs do not have more resale value than fixed/free. If you want to start riding freewheel and gradually move on to fixed, you need a wheel with a fixed/free hub, a fixed cog + lockring and a freewheel. Fixed cog + lockring go on one side of the hub, freewheel on the other and whenever you want to switch you can just turn the wheel around.