Primarily a single equation and a double equation can not be called the same but when you are solving the equations you will find that the last step of both kinds of the equations is the same. The following example can make you understand the concept quite well: If we first take the example of 2x+3y= 7. In this single variable example when we determine the value of x and Y we will find that they stand at two and one respectively. But when we take the example of the equations that takes two different equations to determine the value of x and y we call in double variable equation. Even in this kind of variable we will find that at the last step of the equation we apply the same process as that of the single variable equations. So in that aspect we can treat them as same.