Why do you supposed to have only one independent variable?

When you analyze something, you usually specify two variables, one independent variable, which you can somehow alter or set specific values for, and a dependent variable, in which you see the effect of altering the independent variable. Strictly speaking, you don't have to have only ONE independent variable, but doing so makes the analysis process much simpler. As most phenomena depend on more than one variable, what one does is usually set all other variables as constant and only pick one independent variable. After that, one repeats the whole process choosing another independent variable (and leaving the previous one constant). After analyzing a set dependent variable by testing several independent variables, one can join the results together and get a single relationship between all the tested variables.

A good example of this would be the ideal gas law: PV=nRT. To arrive at this relationship between pressure, volume, number of moles and temperature, scientists had to test pairs of variables to get individual relationships. Only after finding each one could they add them all together to make an equation with four variables and not just two.