Basically there are three questions in nature human tries to ask in his life time. All of theses questions are asked and answered in sequential order: First is “what”: In that we try to understand and know what is our environment the surroundings. what is happening that are sensed by our five sense organs like with eyes what we are seeing, with ear what we are hearing, with nose what we are smelling, with tounge what we are tasting and with skin what we are feeling. Then second is “how”; we try to explain the processes that how after all all such wonderful things are happening. How our body is finctioning and how after all all the natural phenomenons are occuring. And then in last the third question is the ultimate question “why”: There seems no satisfying answer to this big question. Even if we have understood what is happening and how it is happening. In nature so far human have developed or evolved it has not been able to answer most of the whats prevailed in nature. still little it has known about how it is happening and most of the so called advanced research are proving pigmy before this second question itself. Why remains beyond the scope of human endeavor and its logic.
So far so your question is why there are two testicles, the answer can be as simple as why there are two eyes, or two legs or two arms or two ears or say two boobs? If at all further we want to churn our imagination on your insistence, then it can be said that nature has tried to maintain the symmetry and the plan is accordingly to have two testicles. But then it is again an attempt to answer how and not why.
Now coming to the second part of your query, X and Y chromosomes are present in each and every cell of human being whether it is ear, eye, or intestine, liver or say testicles (your paradigm organ). It is only the testicles where 46 chromosomes are divided into two parts each carrying 23 chromosomes and this happens equally in both the testicles. They mix up with semen through vasa difference and carried via epididymis to penis and comes into final ejaculation. Such ejaculations carry both the cells bearing either X or Y. It is a mere chance which one fertilizes the ova and so is the fate of embryo and baby. I hope you have your answer.
I would like to add one more comment for you that it is perhaps in our capacity to answer the questions like what and how in which whole life is finished and still we feel incomplete. To answer why it takes the way of spirituality, which can be felt and perceived by oneself and cannot be answered by other. In the process of understanding this ultimate why we mingle with nature and that is what we human call as heavenly abode or finding the God.