I've never personally made one- But this is how I would start if this is something that you want to create.
-Initially my mind goes to make two separate line charts and combine them onto one dashboard.
- Play with filters and dashboard actions to make the two interact as expected
-To select a range, I would just click and drag
This seems like a very interesting idea and would love to see this come to fruition!
You might be making more work for yourself than you need to. I don't think most people realize that most chart types can be zoomed into, just like zooming into a map. By default Tableau hides the controls for every chart type except maps. But if you right-click a blank part of your chart and then click the last option in the context menu of the chart "Show View Toolbar" you'll get zoom and selection controls:
Now you can use the zoom control to define the two month area you want. Just unpin the view to get to the original view.
Alternative (and this is what I most often do) just use the control-scroll wheel to zoom in. Remember the zoom will center on your cursor so keep it close to the zero line. Then click a blank area and hold for 1-2 seconds. Now start moving the cursor and you'll see that you are now in pan mode. (The cursor symbol doesn't change until you start moving it.) The easiest way to reset to the full view is to turn on the View Toolbar and unpin the view. Alternative you can double-click the Y-axis, click Edit Axis and change it back to Automatic. Then repeat for the X-axis.
If you train your users to use this feature, you'll save yourself a lot of time building custom dashboards using action filters.