If you construct one side of the window frame and cut a 45 degree bevel using booleans, then duplicate it and position it by hand against the other side, you are sure to have a small gap. If I am envisaging the kind of window you are making right, the frames should just butt against eachother with no divider. Giving a standard glass a bump, white diffuse value and a touch of white ambience can make an OK frosted glass look.