I think in depends in part on how big you mirrors will be. I spent a lot of time trying to answer that question earlier this year. Couldn't find a solid answer other than it should be no longer than the vanity and should look right. Not much help.

We have a 48 inch vanity, 42 in mirror, and 30 inch light bar. If you want a pic from a different angle, just ask.

To see how much light it makes, can't take it head on or it just washes the lens out

With the exhaust light on, plenty of light for us!

Just put one light on either side of the mirror.

I don't know the "right" answer to your question, but I can tell you that you need at least 180W of light above each mirror for enough illumination to apply makeup - and even that is a little low. My pet peeve is not enough light in a bathroom, hotels are notorious for this. I don't think one light on either side of a mirror is adequate, unless each bulb is 100W and you have 4 of them over a vanity (two mirrors, 4 lights).