title: Is there a way to merge two form array controls?
body: <p>I have two  form arrays in my code. I need to combine them and perform addition and deletion on the combined array. 
  I need to concatenate the form array controls of each form array</p>
  
  <pre><code>let a = this.nextBillForm.controls["electricityBillCycleEnergyCharges"] as FormArray;
  let b = this.nextBillForm.controls["electricityBillCycleOtherCharges"] as FormArray;
  </code></pre>
  
  <p>1)populate by for each</p>
  
  <pre><code>a.forEach(element =&gt; {
    b.push(element);
  });
  </code></pre>
  
  <p>2) concatenation</p>
  
  <pre><code> a.concat(b)
  </code></pre>
  
  <p>Tried both ways. Both show errors</p>