It might be a good idea to get them identical toys and to ask your friends and family to give them both the same presents, playthings or clothes. At the age of 15 months, their language is just beginning to develop and it would be very difficult to explain anything to them. They will tend to mirror each others actions and the frustration of not getting what the other one has is very real. About two years later, they may voluntarily play with different things, but it is best to leave it to their choice at that time. Enjoy the parenting and let the children behaviour guide you on what to do. There are no formula solutions for anything. What works best is a combination of tender affection, patience and consistency. All the best.