Vue.component('toggle-buttons',{
  props: {
    leftSelectedInitially: {
        type: Boolean,
        default: true,
    }
  },
  data() {
      return {
          leftSelected: true,
          rightSelected: false,
      }
  },
  beforeMount() {
      //this.leftSelected = this.leftSelectedInitially;
      //this.rightSelected = !this.leftSelectedInitially;
  },
  methods: {
      toggle(override) {
          console.log(`override: ${override}`)
          this.leftSelected = override == 'left';
          this.rightSelected = override == 'right';
      }
  },
  template: `
<div role="list">
  <div role="listitem">
      <slot name="left" :is-selected="leftSelected" :toggleLeft="toggle('left')" />
  </div>
  <div role="listitem">
      <slot name="right" :is-selected="rightSelected" :toggleRight="toggle('right')" />
  </div>
</div>
`
});

new Vue({
  el:'#app',
  methods: {
  toggle(direction) {
    console.log(`direction: ${direction}`)
    this.$refs.tb.toggle(direction);
    }
  }
})

#app { line-height: 2; }
[v-cloak] { display: none; }

<div id="app">
<toggle-buttons ref="tb">
    <template v-slot:left="{ isSelected }">
        <button
            class="button"
            :class="{ secondary: !isSelected }"
            :aria-pressed="isSelected"
            :togglable="true"
            v-text="'left'"
            @click="toggle('left')"
        />
    </template>
    <template v-slot:right="{ isSelected }">
        <button
            class="button"
            :class="{ secondary: !isSelected }"
            :aria-pressed="isSelected"
            :togglable="true"
            v-text="'right'"
            @click="toggle('right')"
        />
    </template>
</toggle-buttons>
</div>
<script src="https://unpkg.com/vue@2/dist/vue.min.js"></script>