Is it possible to combine 2D objects to make a solid? I have some complex
polyhedrons that combine, with all faces facing outwards, to make a solid.
However, they don't show in F6. Strangely, a single polyhedron will show in
F6, even though it is not solid, but when the edge coincides with another
polyhedron they both disappear. Here is a simple example of a cube made out
of two polyhedrons:

polyhedron(
points=[ [0,0,0],[10,0,0],[10,0,10],[0,0,10],
[0,10,0],[0,10,10],
[10,10,10],[10,10,0]
],                                // the apex point
faces=[[0,3,2,1,0], [0,4,5,3],[5,4,7,6,5],[1,2,6,7,1],
[0,1,7,4,0]
]
);
polyhedron(
points=[ [0,0,10],[0,10,10],[10,10,10],[10,0,10]
],                                // the apex point
faces=[[0,1,2,3,0]]
);

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Wikipedia: "In elementary geometry, a polyhedron (plural polyhedra or
polyhedrons) is a solid in three dimensions with flat faces, straight edges
and sharp corners or vertices."

Both of your polyhedrons are actually invalid (they aren't solids), which is
why it doesn't work.

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Ok, so I was afraid of that. The problem arises if you want to create a 3D
object, made of many individual 2D polyhedron parts that is only solid when
all the parts are combined. Creating one huge polyhedron would create a huge
list of points and point references in the faces. Designing and
troubleshooting that can be a nightmare. Editing it would be even worse
since all the face references might need to be adjusted.

To address this I created a function to combine multiple individual
polyhedrons into one large polyhedron. This way all the point references in
the individual polyhedron faces will only reference their own points. In
case anyone else has the same issue here is my function. Below is the simple
example from above created from two separate [point, faces] lists. Each
polyhedron can be rendered separately while designing (try it by using
polyhedron(points=poly1[0], faces=poly1[1]) but is combined to make the
final solid. This does F6!

//input is a list of [points, faces]
//For each faces, increment the point references by the previous number of
points
function combine_polyhedrons(list, points=[], faces=[], point_count=0, i=0)

i < len(list) ? 
    combine_polyhedrons(list,
        concat(points, list[i][0]),
        concat(faces, [for (face_i = [0:len(list[i][1])-1])
            [for (point_i = [0:len(list[i][1][face_i])-1]) 
                list[i][1][face_i][point_i] + point_count]]),
        point_count + len(list[i][0]),
        i + 1)
    : [points, faces];
        

//Each poly is [points, faces]
poly1 = [[ [0,0,0],[10,0,0],[10,0,10],[0,0,10],
[0,10,0],[0,10,10],[10,10,10],[10,10,0]
],
[ [0,3,2,1,0], [0,4,5,3],[5,4,7,6,5],[1,2,6,7,1], [0,1,7,4,0] ]
];
poly2 = [[ [0,0,10],[0,10,10],[10,10,10],[10,0,10] ],
[ [0,1,2,3,0] ]
];

new_polyhendron = combine_polyhedrons([poly1, poly2]);
polyhedron(points=new_polyhendron[0], faces=new_polyhendron[1]);

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