I use printable postage exclusively (well, I COULD go to the post office but never want to).  I am sending a requested book, plus one of my unpostables.  Is there a way to print the postage for these two to go together?  Would it just be to put the weight as the combined weight of the two?

Thanks ahead for your answers.

Yep, just increase the weight on the PBS postage page

I tried to do that for a 2 for 1 deal, but it didn't update and I still just got the same amount as if it were just the one requested book.

It depends on the weight of the two books.  See the USPS rates here http://www.paperbackswap.com/help/help_item.php?id=515  Media mail rates go in one-pound increments so it's quite possible to add on a book and not need to increase the postage.

Say the requested book is a hardback that weighs 1 lb 2 ounces.  The postage rate will be $2.77 (plus the DC fee plus the PBS fee). Then your unpostable is a MMPB that weighs 6 ounces.  Your total weight is 1 lb 8 ounces, which is still under two pounds, so the rate is still $2.77 (plus fees).