the only way i know (i deal with this problem myself, frequently) is to print each page to an EPS. since you don't have publisher you'll have to instruct your client how to do this. make sure you cover all the options when printing to file so it works right the first time (embed fonts, proper page size, etc...)
if you want (since this process leaves you with no preview when imported into quark) you can open the EPS' in Illustrator (maybe Freehand, too, i haven't tried) and re-save so that you can edit the documents. this can be a painfully slow process. another option, if you need a preview, is to let Photoshop rasterize the files, instead of Illustrator, opening them at 72dpi/ppi for a smaller file size and then center both the client supplied EPS' in one graphics box (shift>apple>m), duplicate the graphics box right on top of itself (apple>option>d with offsets of x:0, y:0), import your version (i'd recommend using the Photoshop method i described if you don't need to make edits) , group them together, suppressing printout for your version (under the 'modify' menu in Quark).
i'm not sure if you need to send the lo-res/converted version to the back when outputting because i usually delete the preview version because once the document's put together, you don't need it anymore.
WATCH FOR FONT/GRAPHIC PROBLEMS, especially since the files you're receiving are probably coming from a PC.
hope this info is at least of some help to you.
Brad Lakowske
Heinzen Printing and Promotions
heinzenart@charter.net; W
doc_ellis73@yahoo.com; H