There’s no real way to only add the items that are dropping down to a second row to the “hamburger” menu, but you can choose when to show the mobile menu, so you can initiate it at an earlier time than normal.
This CSS should do it: https://gist.github.com/generatepress/c23aef2d05807c39bb32
Just change the 1000px to whatever width necessary for you.
If you need it so only the second row items show up in a mobile menu, there may be a jQuery script out there that does it, but I’ve never stumbled across/heard of it.
Let me know if you have any other questions 🙂