You can use Stamp Papers for property Y provided the seller & buyer of Property Y & Property X are same people. The Stamp Papers are purchased in the name, address &/or other identification of the seller & buyer of the property as this is noted behind each of the Stamp Papers which has the government Stamp affixed & signature of the Stamp vendor, the same facts are mentioned in the register kept by the Stamp Vendor for recording the sale of Stamp papers. The sale deed prepared on the Stamp Papers should bear the name of only those people in whose name the Stamp Papers was purchased & no one else to prove the genuineness of the transaction for all purposes. In case different name appear behind the Stamp Papers then on the Sale deed of the property the Registrar will refuse to sign & stamp these Stamp Papers & ask you to buy new Stamp Papers in the name of actual seller & buyer only. The purpose of all this is that proper Stamp duty being paid for the transfer of property by sale between the parties named in the Sale deed. However, you can get the unused Stamp Papers (of property X) cancelled & reissued in the name of new parties (Property Y) if these were not bearing the name of same buyer & seller by paying nominal fee for this purpose.Answer 2
stamp papers already purchased can very well be used for any other transaction.
if the stamp paper bears your name you are at liberty to use for any purpose which require stamp paper.
here in tamil nadu usually the stamp paper will be purchased in the name of purchaser who bears the stamp duty & registration fee and the seller name will not be there. when a deal is not through in one transaction you can use it for other.
the stam paper can be 6 months old. but the deed execution day shall be just the day or few days before the date of registration.