let input = "зебра, ребра";

let firstWord = input.split(", ")[0];
let secondWord = input.split(", ")[1];

let isThereEqualSymbol = false;

let indexOfTheFirstEqualSymbol = -1;
let indexToPrintFirstWord = -1;

for (let i = 0; i < firstWord.length; i++) {
    for(let j = 0; j < secondWord.length; j++) {
        if(secondWord[j] === firstWord[i]) {
            isThereEqualSymbol = true;
            indexOfTheFirstEqualSymbol = i;
            indexToPrintFirstWord = j;
            i = firstWord.length;
            break;
        }
    }
}

let matrix = new Array(secondWord.length);

for (let i = 0; i < secondWord.length; i++) {
    matrix[i] = new Array(firstWord.length);
}

if(isThereEqualSymbol) {

    for(let row = 0; row < matrix.length; row++) {
        for(let col = 0; col < matrix[row].length; col++) {
            if(col == indexOfTheFirstEqualSymbol) {
                matrix[row][col] = secondWord[row];
            } else if(row == indexToPrintFirstWord) {
                matrix[row][col] = firstWord[col];
            }else {
                matrix[row][col] = " ";
            }
        }
    }

}else {
    console.log("There's no equal elements");
    return;
}

console.log(matrix);