[рони]

IZUM,
вариант для любого количества начальных массивов (все элементы разные)

function repeat(arr) {
            let retry = [],
                temp;
            arr.reduce(
                function(prev, current, index, arr) {
                    if (prev === current) temp.push(index);
                    else temp = [index];
                    if (temp.length == 2) retry.push(temp);
                    return current;
                }
            );
            return retry
        }
        const ms = [2, 8, 13, 15, 19, 27, 33, 36, 43, 46],
            mo = [0, 4, 6, 11, 17, 21, 25, 31, 38, 41, 45, 48],
            mt = [3, 22, 23, 24];
        function merge(...arg) {
            let l;
            if( typeof(arg.at(-1)) == "number") l = arg.pop();
            let s = arg.flat();
            s.sort((a, b) => a - b);
            let arr = s.map(k => arg.findIndex(a => a.includes(k)));
            arr = repeat(arr).map(ar => ar.map(i => s[i]));
            if (l !== void(0)) {
                let i = arr.findIndex(a => a[0] > l);
                let dp = arr[i] || [],
                    dL = [];
                if (i > 0)
                    for (; i--;) {
                        let b = arr[i].at(-1);
                        if (b < l) {
                        dL = arr[i]; break;
                        }
                    }
                return [dL, dp]
            }
            return arr
        }
        console.log(merge(ms, mo));
        console.log(merge(ms, mo, mt));
        console.log(merge(ms, mo, mt, 17));
        console.log(merge(ms, mo, mt, 25));