Yes, if you are buying them both at the same time, then just add the glass to your cart without a design.  We will use the design from the guestbook.
If you have ordered a Guest Book in the past and want to re-use that design you can do that too, just make note of your previous order number in the "Order Notes" field at checkout.
Alternatively, you can email us and we can make sure everything is as it should be.