SOURCE

var vm = new Vue({
  el: '#demo',
  data: {
    firstName: 'Foo22',
    lastName: 'Bar33',
    fullName: ' aa'
  },
  watch: {
    firstName: function (val) {
      this.fullName = val + ' ' + this.lastName
    },
    lastName: function (val) {
      this.fullName = this.firstName + ' ' + val
    }
  }
})

<script src="https://cdn.jsdelivr.net/npm/vue@2.5.16/dist/vue.js"></script>

<div id="demo">{{ fullName }}</div>

	body{
		color:white;
	}