Just my 2 cents: Get as parallel as you can to the glass (to maximize depth-of-field) - of course, this is going to depend on whether you are going to incorporate any other compositional elements (foreground pews, whatever). take a spot meter reading from a pane that you would consider "medium" in tonality and shoot at that exposure - I would stop the lens down to about the middle of its range (best optical results) - and DEFINITELY use a tripod. The dual exposure idea is good too, if there is going to be anything in the foreground to expose for as well (gonna be about impossible to do it in one exposure if you've got something in the foreground unless you bring in some lighting and balance it to the window) If you want the glass to really "pop" as a subject in itself, turn all the light out in the building, exposing for just the light coming through the glass. Something that may be cool - a silhouette of something/someone in front of the glass - but now we're getting into your creative vision - which needs to be all yours.