Diverging Staves

The nearest you will get in MS4 is to use an Ossia, explained in the Handbook in the section "Showing staves only where needed":
https://musescore.org/en/handbook/4/showing-staves-only-where-needed#Os…

You won't be able to have diagonal stave line diverging from a single stave, but you can mark the two staves as Violin I and Violin 2 with a Stave Text (set its style to "Instrument change").

If you really need these diagonal stave lines for notation, perhaps you could do this:
1. Break the system into two parts, by inserting a Horizontal Frame.
2. Create a notation image to branch from the single stave to two staves.
3. Insert the image into the frame.

However, you will still face the problem that the diverging lines need to arrive at the staves which are placed by MuseScore's layout algorithm. So the path of the original stave will remain horizontal, and the path to the ossia stave will go upwards.