SOURCE

let app = new Vue({
  el: '#app',
  data: {
  	first: '',
    last: ''
  },
  computed: {
  	fullname: {
    	//getter function
    	get: function() {
    		return this.first + " " + this.last;
    	},
      //setter function
      set: function(value) {
      	let name = value.split(' ');
        this.first = name[0];
        this.last = name[1];
      }
    }
  }
});

<div id="app">
  <h1>Hello {{ fullname }}</h1>
  <p>First Name: {{ first }}</p>
  <p>Last Name: {{ last }}</p>
  First Name: <input type="text" v-model="first"> <br>
  Last Name: <input type="text" v-model="last"> <br>
  Fullname: <input type="text" v-model="fullname"> <br>
</div>