Can one variable be fed from 2 sources?

I am trying to build a form with two selection boxes of e-mail addresses. The first list is managers, the second list is supervisors.

Using the "multi-page form" div technique, I am showing/hiding one dropdown, and the user can click a button to alternate/swap out the dropdown list.

Is it possible to somehow have both dropdowns feed the same "Dynamic To" variable for e-mail? If so, I am having troubles figuring it out.

Things I have tried:
[list=1]

Letting both lists use the same variable name... this attempt only worker for the 2nd list... as it was defined to use the variable last

giving each list their own variable, use a 3rd variable for the actual "Dynamic To", and somehow shove the list selection into that variable

[/list:o]

The code below follows #2:

<?php
global $user;
$user = &JFactory::getUser();
?>

<h3 style="TEXT-ALIGN: center" align="center">Phone Message Form</h3>

<input type="hidden" name="subject" value="">
<input type="hidden" name="email" value="">
<input type="hidden" name="EmployeeName" value="<?php echo $user->name?>">
<input type="hidden" name="MessageDate" value="<?php $today = date("F j, Y, g:i a"); PRINT "$today"; ?>">

<fieldset>
<legend>Phone Message Details</legend>
<table cellSpacing="0" cellPadding="0" border="0" id="table1">
	<tr>
		<td><b>Message for:</b></td>
		<td>

<!-- begin page one -->
<div  id="mainForm_1"> 

<!-- FORM FIELDS FOR PAGE ONE HERE -->

		<select name="email1">
			<option selected value="">--- Select Message Recipient ---</option>
			<option>email@address.com</option>
			<option>email1@address.com</option>
			<option>email2@address.com</option>
			<option>email3@address.com</option>
			<option>email4@address.com</option>
		</select>
		<input type=button onclick=" collapseAll(); expandElem('mainForm_2');" class="mainForm" value="Show Others" />

</div>

<!-- begin page two -->
<div  id="mainForm_2"> 

<!-- FORM FIELDS FOR PAGE TWO HERE -->
		<select name="email2">
			<option selected value="">--- Select Message Recipient ---</option>
			<option>email10@address.com</option>
			<option>email11@address.com</option>
			<option>email12@address.com</option>
			<option>email13@address.com</option>
			<option>email14@address.com</option>
		</select>
		<input type=button onclick=" collapseAll(); expandElem('mainForm_1');" class="mainForm" value="Show Original" />
</div>

		</td>
	</tr>
	<tr>
		<td><b>Caller's Name:</b></td>
		<td><input type="text" name="CallName" size="50" value=""></td>
	</tr>
	<tr>
		<td><b>Company:</b></td>
		<td><input type="text" name="CallCo" size="50" value=""></td>
	</tr>
	<tr>
		<td><b>Location:</b></td>
		<td><input type="text" name="CallLoc" size="50" value=""></td>
	</tr>
	<tr>
		<td><b>Phone Number:</b></td>
		<td>(<input type="text" name="CallPh1" size="5" value="">) <input type="text" name="CallPh2" size="5" value=""> - <input type="text" name="CallPh3" size="5" value=""> Ext:<input type="text" name="CallPh4" size="5" value=""></td>
	</tr>
	<tr>
		<td><b>Email:</b></td>
		<td><input type="text" name="CallEmail" size="50" value=""></td>
	</tr>
	<tr>
		<td><b>Reason For Calling:</b></td>
		<td><textarea name="CallReason" cols="50" rows="10"></textarea></td>
	</tr>
</table>
</fieldset>
<fieldset>
<legend>Submit</legend>
<center><input type="submit" value="Send Message"><input type="reset" value="Reset Form"></center>
</fieldset>
</center>

... and my attempt to shove the selected list item into the variable was in the following On Submit code:

<?php if (is_null($_POST['email1'])=0) {$_POST['email'] = $_POST['email1'];} ?>
<?php if (is_null($_POST['email2'])=0) {$_POST['email'] = $_POST['email2'];} ?>

Am I way out to left field?

I figured it out... apparently the is_null() function was stopping me.... changed my On Submit to:

<?php if ($_POST['email1'] != null) {$_POST['email'] = $_POST['email1'];} ?>
<?php if ($_POST['email2'] != null) {$_POST['email'] = $_POST['email2'];} ?>

...and everything worked!