Start out making your wall calendar and go to the check out (don't finish checking out) just get to that point, then click the continue shopping button on the bottom. Go on sidebar and choose calendars again, but pick the desk one. Customize that one, add it to your cart and check out.
The only thing that you pay for is the shipping (because both calendars are FREE). I just finished my order and for both calendars together it was only $6.89. So you can get 2 calendars for $3.45 each.