It was quite an interesting task to figure out. You can put the following in a button to test it out:

"takes the selections from the multiple choice fields
and creates a string for each combination of unit/floor
selected";
let unitArray := chosen(unit);
let floorArray := chosen(floor);
let newCombo := for loop1 in unitArray do
        for loop2 in floorArray do
            loop1 + "/" + loop2
        end
    end;
"turns the string into an array. We love arrays";
let x := split(concat(newCombo), ",");
"get all records from Table2";
let getAll := (select Table2);
"find all matching records";
let matchFloor := for loop1 in x do
        let findSlash := index(loop1, "/");
        let fir := substr(loop1, 0, findSlash);
        let sec := substr(loop1, findSlash + 1);
        let withFloor := getAll[unit1 = fir and floor1 = sec];
        withFloor.number(Id)
    end;
"takes results from above and compares it
to all records and creates an array
of records that exclude the array from above";
let noMatchFloor := getAll[var n := Id;
        count(matchFloor[= n]) = 0];
"deletes the new array";
delete unique(noMatchFloor);
"create records for unit/floor not existing
in Table2";
for loop1 in x do
    let findSlash := index(loop1, "/");
    let fir := substr(loop1, 0, findSlash);
    let sec := substr(loop1, findSlash + 1);
    let unitFilter := getAll[unit1 = fir and floor1 = sec];
    if count(unitFilter) = 0 then
        (create Table2).(
            unit1 := fir;
            floor1 := sec
        )
    end
end

If everything works like you want, you can move the code into the Trigger after update for the floor field.