I know this topic is old, but I do have something to add. Hopefully my thoughts will help Widower 52 and anyone else faced with the same situation.

This is not an uncommon issue, yet it is one that many people seem to have a real problem with, for it is very emotional. I feel it is a topic that needs to be discussed on an ongoing basis. Everyone will have slightly different opinions, some based on emotions, and some based on spirituality.
When we are on Earth, our loves, concerns, fears, desires all come from the physical/emotional part of us. It is hard to imagine the way things will be in the afterlife. If one does not believe in the spirit and an afterlife then who is buried where does not really matter because there would be no awareness of the issue after death.
I still believe that one should give all their attention to their current spouse/marriage and make that relationship the best they can. Assurance that the current spouse is the most important person in ones life is necessary for a good marriage.
Being a spiritual person, I believe that the first wife is in a peaceful place and has a spiritual bond to her husband, the man she had children with, the only man she was married to. This spiritual bond can never be broken by anything on Earth. The promise made to her should be honored, not for her sake because she is not in the grave -- she is in spirit. The promise to her should be honored for her children's sake. That was a family unit that at one time never thought they would ever be parted. To not honor that memory and promise would be telling the children of that marriage that the promise made to their mother has been broken.
It is still very strong on my mind that if Widower 52 dies before his current wife does and for the sake of peace while alive promised to be buried by her, then what happens if the current wife remarries? Would she face the same issue with her new husband? Would Widower 52 be left at a grave that is neither by her or his first wife? How would the children and any grandchildren of the first marriage of Widower 52 and his first wife feel? They would not be able to visit both graves together to honor their parents.
Obviously, the decision comes down to what Widower 52 decides. It is his decision alone. There are a lot of factors to consider.
I believe that the current wife does feel like "second best", but if she truly loves Widower 52, she will honor his decision and respect his promise to his first wife. Also, very obviously, it is not a wise thing to take the current wife to the same places for vacations/trips where he and his first wife spent time and made memories. The current wife wants and needs to make her own memories with Widower 52 and not be overshadowed by memories of another.
When we have walked on from Mother Earth, our spirits will reunite with all the loves of our life and bond to the one who is our twin soul or the soul mate we have always loved the most. In spirit, we do not have the base human emotions such as jealousy. It is a higher level.
It is the ones left behind who will need to get through their bereavement in peace. With that being said, I think Widower 52, or anyone in this same situation, needs to sit down with the children he and his first wife had and discuss it then come to a decision.