"Ladies and Gentlemen" is a common expression used to address an audience or crowd. While a crowd may consist of only gentlemen or only ladies, or possibly even just one gentleman or lady, it's entirely acceptable to address them as such anyway, as they will 'get' what you mean.
If you want to be 'correct', you could welcome them as "Sir and Madame" if it is one man and one woman. For a room of all one gender "Gentlemen" if it is all gentlemen and "Ladies" if it is all ladies, though you should only do this if you are certain that this is the case. Even then, it is unlikely that "Ladies and Gentlemen" will be objected to.
From related Wiktionary entry:
ladies and gentlemen pl (plural only)
(idiomatic) Used to address an audience.
- Use is so idiomatic that even unisex audiences are sometimes addressed this way, though "ladies" or "gentlemen" would be more correct.
- The forms "lady and gentlemen" and "ladies and gentleman" are rarely used even when strictly correct.
- Nearly always used with "ladies" before "gentlemen", as opposed to "gentlemen and ladies".