I also prefer one large mirror. I think the smaller mirrors make the space look choppy. Personal preference for sure!

I prefer one, the larger the better.

I prefer two framed mirrors, especially with your light set-up.

I prefer two mirrors as well because I think it gives a more balanced look and it provides defined spaces, his and hers in our case. Your statement about wanting to make it so that 3 people can use it is what has me curious though, is this a what if type scenario or is a 3rd person actually going to be in there all the time? The reason I ask is because DH and I used to make choices based on the what if's a lot. We'd spend the money or compromise on what we really wanted only to never have those "what if's" materialize. We bought a crew cab truck so if we needed to we could tote people but never once did anyone sit back there. We turned a room (that could have been my craft room) in to a guest room and in 10 years it's only been used a handful of times and an air mattress would have worked just as well. So if you want the big mirror because it will actually be used in the context you're planning for, great...then there's a need and that would be the more functional choice for your situation, but if you're just thinking well maybe 3 people might need it, then don't focus on that and do what is more practical for your every day living. KWIM?

(And Ida is right, about the two lights)

I like both, but with your set up, the single mirrors will be fairly narrow for the space because of how close the sinks are to the ends of the counter. Sometimes that is balanced out by a center wall sconce, when you have wall mounted sconces but it will be blank space with your set-up. I would do a mock up with painter's tape or newspaper or cardboard to mimic the size you'd need and see how it looks.

It seems it could be awkward to have framed mirrors since the lights are on wood already. I'd fill the area with an unframed single mirror.

The "80's" style of filling that space with mirror is the most functional..particularly for children. The one or two question is one of style. Two is vogue right now. Which will serve you better--style or function?

I am with the others. Either will work. I am leaning toward the double mirrors though. I think two mirrors in a burnished metal frame like your faucets would work beautifully. Uttermost have a wide selection. Good luck.

In a larger and/or master bath, I might do two. The space is small and narrow and already has a lot of details happening -- not just the wood behind the light fixtures, but the wood window frame and lots of tile patterns. It's a guest bath, and defining his/hers spaces is really not an issue. Oly is right about the placement so close to the side walls. I would go with a single mirror.

I hate cleaning large mirrors so I would do two separate for ease of cleaning and for no other reason. I clean kids bath mirrors daily and a large glass plate is a nightmare to me.