You can instruct XO to start a script or bat file instead of X-Plane. This way you can start multiple applications at the same time. Please note that you will have to add X-Plane to the list if you want to start it too.
In chapter 13 of the manual all the steps are described.
The manual describes the steps for a bat file, but it work the same for scripts. One important remark: provide the full path to the script or bat file. XO will not specifically look in its own folder or in the X-Plane folder.