Maxwell doesn't mind overlapping geometry. I don't know if using 2 planes will give you desireable results, best way to tell as Maxer stated above is to just try it. Render one image with the glass as 2 planes and then another with glass as one plane with thickness... compare which one looks better as well as how long they each took.
As for a glass material the built in mxm wizard can create glass and/or AGS. If you have a specific problem with the glass mat let me know.