Hi Luuk Thanks for commenting in our community! Unfortunately inserting double lines is not an option. I understand how this would be useful! Would you mind adding your idea to our feature request form?
As a possible workaround to achieve double lines you will have to manually draw a second line to follow the first line. Depending on the shape of your first line it might not line up perfectly. You will then select the desired line and pick the color. Here is quick screenshot of what I created. Hope this helps! Let me know if you have any questions.