By default settings of QS you cannot have logo next to menu after scroll down. But using following solution it can be done:

1. First step, reduce logo dimensions to small one, for example use 30px of height for logo image.
2. Upload smaller PNG logo image to server to: /image folder
3. Create/Add new menu item
   a) Type: External URL
   b) Link: put here URL of your site
   c) Link Image : choose smaller logo image
   d) Add Menu Title : No
   e) Set as a first menu item , in order list
   f) Custom CSS Class: scrollogo
       - before class name must be one space.
4. Use following custom CSS inside custom.css file:

div.sticky-wrapper .sp-menu-item.scrollogo > a {display:none;}
 div.sticky-wrapper.is-sticky .sp-menu-item.scrollogo > a {
 height: 30px;padding-top: 10px;display: block;
 }