Gifts that are on hold in another Gift Giver's cart will expire about 48 hours if the purchase is not completed. So if you're holding on to them, don't wait too long! On the other hand, if you're eyeing something in someone else cart, you're welcome to keep checking back to see if they ultimately get marked as purchased or if they become up for grabs.
To avoid duplicate gifts being given, it's best not to purchase gifts that are on hold.