Creating diptychs in Lightroom is fairly simple and having received an email from someone inquiring how I accomplish these when I alludeded to how I rarely ever use Photoshop, his interest peaked.
Rather than walking you through a whole written tutorial, I could simply make the preset which I use, downloadable to you. There’s a landscape and portrait version. How do you install it? Matt Kloskowski from Killer Lightroom Tips has a quick video tutorial on how to get it running.
How do you use it? Assume you already installed the preset, once you’re in Library Module in Lightroom, select the 2 photos that you want to convert into a diptych (they don't have to be the same orientation), then go to the Print Module and on the left side you’ll see all your presets and hopefully the one you just installed as well. Select either the Horizontal or Portrait one.
To export the diptych, while still in the Print Module, click on the Print to File tab and it will save it as a jpg for you. Hope this helps.