C., I take Metformin for both metabolic syndrome & PCOS, and have for a number of years. I take 850mg 2x a day, with meals when I remember.
My guess for the split dosing in your case might be that people are less active in the evening & the body might need more "help" from the drug due to the inactivity as we sleep. That is just a guess however, I wasn't able to find anything online addressing the dosing.
While Metformin is used to help regulate insulin and is often prescribed to diabetic & prediabetic patients (like myself), it also helps to regulate other hormones, which is why it is often prescribed (and successful) for treating infertility in women with PCOS.
I did quite a bit of research on this drug for a friend who was TTC a few years ago. Unfortunately, the medical journal articles & studies are not easily found again online, but here are links to a few sites that have info supporting the use of Metformin in your situation.
http://www.lef.org/protocols/female_reproductive/polycyst...
(See specifically the yellow box on Metformin)
http://health.howstuffworks.com/pregnancy-and-parenting/p...
http://www.webmd.com/women/metformin-glucophage-for-polyc...
FYI, Wild Woman commented below regarding the safety of taking the drug prior to/during pregnancy. The first link I posted references that recent studies have shown no teratogenic effects, via studies in 2002 & 2010. Here is a link with more technical information regarding it's use during pregnancy
http://www.medscape.com/viewarticle/515581
Class A has been proven safe in humans & animals. Class B may have adverse effects in -animals- but is safe in humans, or is safe in animals & there haven't been adequate studies in humans.
These links explain the classification of drugs & Metformin specifically.
http://en.wikipedia.org/wiki/Pregnancy_category
http://www.drugs.com/pregnancy/metformin.html
As with all things in medicine, new studies come out to update information, and every doctor has their knowledge & opinion on the safety of what they prescribe.
If you have any questions about taking the drug prior to or during pregnancy, make sure to talk to your doctor & pharmacist about this. If you are unsure, it is always your choice to proceed with any treatment or not.
Best of luck! T. =-)