Vue

HTML

<div id='a1' v-cloak>
  <button @click='x++; shared.y++'>Inc</button> {{x}}, {{shared.y}}
</div>

<div id='a2' v-cloak>
  <button @click='x++; shared.y++'>Inc</button> {{x}}, {{shared.y}}
</div>

Vue

var shared = {
    y: 0,
}

var vm1 = new Vue({el:'#a1', data:{x:0, shared}})
var vm2 = new Vue({el:'#a2', data:{x:0, shared}})