You can use them together but not necessarily one the same element.
For instance I can place a div in a CSS-Grid parent container (which has display:grid) but the child div could have display:flex to lay out it’s children.
Alternatively, you can apply Flexbox and CSS Grid to an element and, if a broswer supports one but not the other then the supported display option would be used. If both are supported, then the last one stated would apply.
In other words, depending on which browsers you wish to support, you can fallback to flexbox if CSS-Grid is not supported (and vice versa).
CSS Grid should be used for 2-dimensional layouts vs. Flexbox should be used for 1-dimensional layouts. Ex) Grid should be used to lay out the sections of a web app (header, navgiation, aside, main, footer). Flexbox could be used to position navigation items within the