If you were to merely glance at the images on these four cards, they wouldn't appear to have anything in common. If you were to read all four entries, however, you'd discover that they are all about relationships -- each of the cards brings two principles face-to-face with one another, and we get to observe them dance as they mingle or repel.
Everyone knows what numbers mean when it comes to counting, but what do they mean beyond that, and why? Since the dawn of mathematics, numbers have been seen as mystical and poetic entities. Two will never be merely "1 + 1". Two has its own world of psychological and spiritual associations that have become embedded in our consciousness over the millennia. You don't have to believe in numerology or Kabbalah to understand that, it's written into our very mythology, even our superheroes with their secret identities.
If the number one represents the unified totality of all things ("All is one" ... "All for one, and one for all!" ... "All 4 One"), then two represents the principle of duality. Maybe duality is the result of an internal schism. Maybe it's a matter of that self-contained whole catching a glimpse of its own reflection, a la Narcissus. Maybe it's an encounter with unaccounted-for entity, some sort of Other. Whatever the cause, that moment when One is no longer alone anymore is something we each have an innate understanding of, and the cards you see above examine that moment from all sides, from the perspective of each of the four elements we discussed before. Is it any wonder that the fiery, ambitious Wands would treat such a schism as a challenge to one's dominance? Or that the emotional Cups would sense an opportunity for a loving relationship?
I touched on this in my post about The Emperor and his yin/yang relationship with The Empress, one of the Tarot's many examples of duality. It gets more complicated as two become three, and three solidifies into four, and four shatters into five, and so forth... Meditating on this initial division/separation/reflection is going to be a huge help in understanding the other numbers as they pile up -- and even if you don't quite understand, you'll at least begin to see that the cards of the minor arcana amount to much more than a bunch of random ideas tacked on arbitrarily to whatever numbers seemed convenient at the time.