These two phrases share similar meanings and emphasize that David and the people of Israel are family. (See: [[rc://en/ta/man/translate/figs-doublet]])
The second phrase defines the first phrase. The "ark of the covenant" is spoke of as Yahweh's footstool to picture God on his throne, with his feet resting on his footstool, which represents all that is in submission to him. (See: [[rc://en/ta/man/translate/figs-parallelism]] and [[rc://en/ta/man/translate/figs-metaphor]])