Newbie Questions

We use InfoPath 2010 to design form and require digital signing.  I set Certificate services as this link:
http://marcellotonarelli.wordpress.com/2009/10/28/domain-certificate-authority-signing-infopath-2007-forms/ (use code signing template)
The form xsn is configured to allow co-sign (multiple users can sign)
We asked two internal users (only for domain internal userA and userB) to digitally signed and it can not validate signature 
each other.  We do not have sharepoint server and users use InfoPath filler to open it.  Users A signs the form and digitally signed and then send the form to user B through email 
When userB opens the form (attachment) in Outlook (every user has InfoPath installed), userB can not see the userA certificate info and the message shows "the signature is not valid" 
these certificates are issued by our internal CA and our Certificate server is in everyone's trusted root certificate store
(details: userA requested the cert from our internal CA and install it and  appears in the user personal certificate store; then digitally sign the form by using his certificate; email the 
signed form to user B.  User B open the form and got" the signature is not valid".  If I export the userA certificate and import it to userB's trusted store and it will work.  This defeats 
internal CA purpose and we have about 1500 users and I can not export and import each user cert.  

Should internal CA validate internal user certificate without exporting and importing each user certificate?  
What am I missing?  is code signing template sufficient for digital signatures? or any security settings I need to change

Thank you for your help.

Newbie Questions

internal CA for InfoPath digital signatures (big help from you)