Janis - this is a question that rears its ugly head about every week or so on the Grumble. Do a search using the words "two-sided" or "both sides" and you will find a treasure trove! As a matter of fact, I brought up the subject in a thread entitled "Encapsulation Problems" about a month ago and a lot of good ideas were discussed about just this problem.
Just be sure to impress your customer about the importance of matting to keep the document away from the glass!
it will make a huge difference whether your customer wants merely to be able to turn the frame around a look at the back (and I suspect they hardly ever really do that) or actually display both sides by suspending the frame in the middle of the room or whatever.
The former might mean hinging or encapsulating the document between two identical mats and using a second piece of glass or - preferably - acrylic instead of a solid backer.
The latter is a nightmare, unless the item will be small-enough to be a tabletop display. I've had skinny acrylic boxes fabricated, just thick-enough to accommodate the afore-mentioned pair of mats, with an opening in the bottom to slide the item through. The acrylic box can be fitted into a slotted acrylic or wood base and viewed from either side.
Documents are perfects candidates for facsimile
framing. We appreciate them for what they tell us
and not for their color, composition, or forms.
Here, a good copy could be made and that could be
set in a window mat that show in the front of the
frame. The real document could be set in another
window mat, using paper edge strips, with its back side showing and this can be set behind the
other mat with acrylic sheet replacing the backing
board. This would allow both sides of the document
to be seen, without exposing it to light.
I'm working on 2 of these now. The documents will be suspended between two identical mats with glass front and back. I'm using a frame with a 9/16th deep rabbet. After measuring the thickness of the package (glass, mats and document) Cut a slot in the rabbet to accept the flat part of a fillet and paint any exposed unfinished surfaces on the back of the frame. Then join the top (or bottom) of the frame with the sides. Insert the package and three legs of fillet. Add the fourth leg of the frame and fillet and secure with small brass screws. The fillet holds the glass without the ugliness of points.
For occasional viewing of the back, use a saw tooth hanger. To display the back, I have added brass feet for desktop use. Also on a Multi-page letter I hinged three frames together to form a multi-paneled screen-like display.
I recently did one where the customer wanted the deckle edge of a print to show and there was information on the back which he would like to be able to see. We floated the print on rag board with mulberry hinges and a spacer to keep it off the glass. A beveled opening was cut on the back of the rag board used to float the print to reveal the information on the back. Fortunately we did not need to reveal the entire back of the print and had plenty of space to hinge. Used UV on the front and reg. clear for the back glass. The frame was a Nielson metal with enough space for the whole package.