The natural world is a complicated place. I've never heard of any reasonable scientist or philosopher of science claim only one theory is needed to understand a phenomenon. Theories, like words in a paragraph, support each other in order to function.
Some scientific theories are more important than others, such as the theories of relativity, quantum theory, the theory of the cell, and evolutionary theory, but the external universe (as opposed to our understanding of it) is a large, complicated place, and no one theory could possibly explain, predict, and describe nature.
Now, that being said, sometimes multiple theories are REPLACED by a single theory. This is known as theory reductionism. A classical example of that is the division between organic and inorganic chemistries, where were presumed to be different sciences since organic compounds were presumed to have a vital force which animated them and gave them biological properties. Currently, it is understood that organic and inorganic compounds are both subject to the same general rules which govern atoms in the periodic table. Another example of two theories would be Galileo's work on bodies on earth and Kepler's work on orbits which were shown to be equivalent roughly under Newtonian mechanics (which were then later shown to be a special case of relativistic physics).
In these cases, sometimes two theories which appear to show different phenomena are really two differing aspects of the same phenomenon.
Two modern philosophical theories which address at a metaphysical level the relationship between our theories and the external world are transcendental idealism and philosophical realism, the former claiming the external world is relatively unknowable, and the latter disagreeing heartily on many points, except that naive realism is an untenable philosophical position.