Simply put, you can't transfer your friend's 250 voucher to your a/c or vice versa via giftmate, its not possible.
But If you want to buy something from indiaplaza.com for say Rs. 500, you can buy 2 indiaplaza gift certificates (select email GC) of Rs. 250 each using both your's and your friend's a/c. You'll get the GC through email within 3-4 days. Later, when you buy from Indiaplaza select GC as the payment option and put both the GC numbers in the box (seperated by comma). This way you can use both your A/C for the same purchase. Your GC will have a validity of 90 days.