[xsl] Validating a XML with 2 schemas, both required

[Josir <josir@xxxxxxxxx>]

Hi folks,

I am learning how to do schemas and I'm doing fine since I tried to 
merge two xsd.

I want to force that my XML files uses a md5 signature described on 
http://www.w3.org/2000/09/xmldsig#

The XML below works fine:

<?xml version="1.0" encoding="UTF-8"?>
<Lista_AP_MS xmlns="www.media-systems-sc.com.br/namespace" 
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"; 
xsi:schemaLocation="http://www.media-systems-sc.com.br/namespace 
AP_MS.xsd http://www.w3.org/2000/09/xmldsig#";>
<AP_MS_Request>
...
</AP_MS_Request>
</Lista_AP_MS>

But this construction accepts but it does not require the <signature> tag.

How can I define the xsd to require both elements ?
Is this strategy the best one ?

Thanks in advance,

Josir Gomes
Rio de Janeiro - Brasil