Hi
Just came across this post so thought I would post what I did.. Sorry if I am bumping it up.
If you just want to have the one login, being the WHMS login, try doing below.
1. Go to Module Manager - new - custom HTML
2. add
<form id="frmlogin" action="billing/dologin.php?goto=clientarea" enctype="application/x-www-form-urlencoded" method="post">
<table class="frame" style="margin: 0 auto;" border="0" cellspacing="0" cellpadding="0" align="center">
<tbody>
<tr>
<td>
<table border="0" cellspacing="0" cellpadding="5" align="center">
<tbody>
<tr>
<td class="fieldarea" width="20" align="right">Email:</td>
<td><input style="width: 102px; height: 22px;" name="username" size="13" type="text" /></td>
</tr>
<tr>
<td class="fieldarea" width="20" align="right">Password:</td>
<td><input style="width: 101px; height: 22px;" name="password" size="14" type="password" /></td>
</tr>
<tr>
<td class="fieldarea" width="20" align="right"><input name="rememberme2" type="checkbox" value="on" /></td>
<td>Remember Me</td>
</tr>
<tr>
<td class="fieldarea" width="20" align="right"> </td>
<td><input type="submit" value="Login" /></td>
</tr>
</tbody>
</table>
</td>
</tr>
</tbody>
</table>
</form>
3. Make sure the 1st line is pointing to your whmcs folder (billing, payments, etc)
4. Press save and your done.
