Changing a single-bowl sink to a double-bowl sink is not a difficult task. It's actually quite a smart DIY job, which will give you much more usable space. Basically all kitchen sinks, regardless of the number of bowls they have, and unless you have a specialty size, will be 33 inches by 22 inches, so a single-bowl sink can easily be replaced with a double-bowl. Bathroom vanity tops will vary in length and depth, but the installation is basically the same as with a kitchen sink. The major difference is that bathroom drain pipes will be 1 1/4 inches and kitchen drains will be 1 1/2 inches in diameter.
Remove the old sink by loosening the screws that are running around the perimeter underneath the sink. Once these screws are removed, you should be able to simply lift the sink out of place. If you are removing a bathroom sink, then just pry the sink off of the vanity.
Set the new sink in place. If you are installing a kitchen sink, tighten the clips on the side with a screwdriver, locking the sink in place. If you are installing a bathroom sink, apply an adhesive caulk where the vanity and the vanity top meet.
Install the drains into the basins of the sink, applying a thin layer of plumber's putty where the drain meets the sink. Plumber's putty will help ensure a watertight seal.
Connect the sink tailpieces or extension tubes (these are long and help extend the height of the drain piping) onto both drains, tightening the slip nut to ensure a proper seal. You may have to trim either the sink tailpiece or extension tube, depending on the distance from your sink to your P-trap.
Connect the sink tailpieces or extension tubes into the outlet drain tee (this lets both drains drain into one pipe), threading the slip nut down onto the outlet tee's threads. You will want to set the center of the tee directly between the two bowls.
Insert the bottom end of the tee into the P-trap (this is already in place from your old sink), tightening the slip nut.