its listed under each special material how it works with double weapons if it doesn't say then u can't do it.
This iron, mined deep underground and known for its effectiveness against demons and fey creatures, is forged at a lower temperature to preserve its delicate properties. Weapons made of cold iron cost twice as much to make as their normal counterparts. Also, adding any magical enhancements to a cold iron weapon increases its price by 2,000 gp. This increase is applied the first time the item is enhanced, not once per ability added.
Items without metal parts cannot be made from cold iron. An arrow could be made of cold iron, but a quarterstaff could not.
A double weapon with one cold iron half costs 50% more than normal.
Weapons made of cold iron cost twice as much to make as their normal counterparts. Also, adding any magical enhancements to a cold iron weapon increases its price by 2,000 gp. This increase is applied the first time the item is enhanced, not once per ability added. A double weapon with one cold iron half costs 50% more than normal.
if its a home game ask your DM to allow the same for Adamantine(half price), if your in PFS you can't do it. also incase you misread when it says 50% more than normal it means half what a full item would cost. so to enchant a double weapon with one head that's cold iron it costs an extra 1000g ontop of the normal price instead of 2000g. and when crafting it costs 50% more than a normal non-cold iron weapon of that type instead of double.
