How wide is the cabinet below the current sink?

We don't know what you have now, but generally, you need to extend the water lines and install a drain system for a double sink. You may be able to install a stop with a double out fitting for the hot and cold and then connect the second drain back to a common trap. Not a really big project but you will need a new countertop and possibly a vanity measuring 54-60" or so. How big is the existing vanity? and will the drawers be in the way?