Volun, как вариант...

<form method="post" id="form_name">
	<input name="first_name" placeholder="First Name" class="form-control" data-i18n="">
	<input  name="last_name" placeholder="Last Name" class="form-control" data-i18n="">
	<input  name="email" placeholder="Email" class="form-control" data-i18n="" value='tmp@test.ru'>
	<button type="submit" id="btn_name" class="btn-formnav btn"><span data-animation-name="next-span"><span data-i18n="">NEXT</span> &gt;</span></button>
</form>
<br>
<form method="post" id="form_pass">
	<input name="password" placeholder="Password" class="form-control" data-i18n="">
	<input  name="pass_conf" placeholder="Confirm Password" class="form-control" data-i18n="">
	<input name="hidden_email" class="form-control" data-i18n="" style="display: none;">
	<button type="submit" id="btn_pass" class="btn-formnav btn"><span data-animation-name="next-span"><span data-i18n="">NEXT</span> &gt;</span></button>
</form>
<script type='text/javascript'>
	var oi=document.querySelector('#form_name [name="email"]');
	var o=document.querySelector('#form_pass [name="hidden_email"]');
	o.value=oi.value;
	document.querySelector('#btn_pass').click();
</script>

Может посмотреть в сторону легких MVVM?

https://codepen.io/sup3rz3n/pen/jJJzyY

const { h, app } = hyperapp;
/** @jsx h */

const state = {
  form: 0,
  fields: {
    username: '',
    password: '',
    email: ''
  }
};

const actions = {
  changeForm: id => state => ({ form: id }),
  changeField: e => state => ({
    fields: {
      ...state.fields,
      [e.target.name]: e.target.value
    }
  }),
  submit: e => state => {
    e.preventDefault();
    alert(
      `submit ${state.fields.username} ${state.fields.password} ${state.fields.email}`
    )
  }
};

const view = (state, actions) => {
  if (state.form === 0) {
    console.log(state.fields.username)
    return <div>
      <input type="text" name="username" oninput={actions.changeField} value={state.fields.username} />
      <input type="password" name="password" oninput={actions.changeField} value={state.fields.password} />
      <input type="email" name="email" oninput={actions.changeField} value={state.fields.email} />
      <button onclick={() => actions.changeForm(1)} disabled={
        state.fields.username === '' ||
        state.fields.password === '' ||
        state.fields.email === ''
      }>next</button>
    </div>
  } else if (state.form === 1) {
    return <div>form2 {state.fields.username}
      <button onclick={actions.submit}>complete</button>
    </div>
  }
};

const main = app(state, actions, view, document.getElementById('app'));

ksa, спасибо, ваше решение помогло.