First, you can take the original sketch and use it facing two directions. Meaning that you can do the left page as the original sketch looks and then flip, mirror, or turn it, to use for the right side of the layout. Here is an example from Tami "Practically Perfect" using Sketch #52.
Here is another layout of Tami's where she turned the page to create a two page layout "Part of our World"
Now the 2nd way is the Stretching method. This works great for most of the sketches but you may have to add extra photo placement or embellishments.
This is one of my favorite pages that Tami has stretched "Not Today Zurg!" using Sketch #22 Now the sketch just looks like its missing half!
And here are a couple more of examples using the sketch stretching "Spring in Concord", "Duck Tales" and "Plutos Corner"