export function Thing {
  constructor(one, two) {
    this.one = one;
    this.two = two;
  }
}

let t1 = new Thing(1, 2);
let t2 = new Thing('one', 'two');
let t3 = addThings(t1, t2); // {one: '1 & one', two: '2 & two'}

function addThings(lhs, rhs) {
  let one, two;
  if (lhs.one && typeof lhs.one === 'number' && rhs.one && typeof rhs.one ==v 'number') {
    one = lhs.one + rhs.one;
  } else {
    one = `${lhs.one} & ${rhs.one}`
  }
  if (lhs.two && typeof lhs.two === 'number' && rhs.two && typeof rhs.two === 'number') {
    two = lhs.two + rhs.two;
  } else {
    two = `${lhs.two} & ${rhs.two}`;
  }
  return new Thing(one, two);
}