how to copy contents from one textbox in one form to another textbox in another form through JS

<form action="formoneaction" method="post">
<input type="text" id="t1" />
<button type="sumbit" class="btn">Insert</button>
</form>

<form action="formtwoaction" method="post">
<input type="text" id="t2" />
<button type="sumbit" class="btn">Insert</button>
</form>

<script>
var t1 = document.getElementById('t1');
t1.onkeyup = t1.onchange = function() {
document.getElementById('t2').value = this.value;
};
</script>

How can I get the script to work when the two elements are in separate forms? The script works when none of them are in forms.

I've never learned JS before so I may not understand very well when someone explains a bunch of jargon. Sorry but I'll try my best!