In KiCAD - is it possible to have two nested rings of copper fill?
KiCad only supports one board per file.

Optimal use of available space is up to the manufacturing house, they can tell whether it is possible to place small boards inside larger ones, because they know the size of the router head they use to separate the boards.

The alternative is to connect the boards, and define V-Grooves to separate the boards later. This could be more expensive to fabricate though, and concerns about the router head side still apply. For this, your board outline should be a single line, and the board should be connected still.