If you want to use a medicine cabinet I would probably stay with the 24 inch and use side lights. You'll need to keep in mind that a bar light above the medicine cabinet will need to extend enough to clear the depth of the cabinet, so finding such a light may be difficult. There are cabinets that have built-in lights so you may want to look at those. Check these out. If you choose a cabinet with built-in lights, I'd go with a wider than 24" cabinet. Other option: keep the light fixture you have and put in a framed, or unframed mirror that is a little wider than the lights. Here are some visuals.

Many people choose vanity lights that are about 75 percent the width of the mirror and center the light above the mirror. You can also choose to use a bath bar light that is exactly as wide as the mirror. An LED mirror is another choice. https://www.lightsonline.com/learn/lighting-ideas-by-room/bathroom-light-fixtures-guide 

I would do a mirror the size of the vanity, this way the entire surface is usable. as to the light where is the outlet?

Hello! Here are some of our examples for 60 inch double vanities with two mirrors. I have attached some pictures for reference on some of our vanities, mirrors, and medicine cabinets. The LED medicine cabinets are 24" and they are very popular! Feel free to click the links below to check out more options! Hope this helps! Click HERE for Vanities The oval mirrors are 36 x 24 inches. The rectangle mirrors are 32 x 24 inches.