Which webmailer you want to use is your own decision.
I'd use only one of them. Because your customers will have questions on the webmailer and will need support in using it. If you offer them two different mailers you'll have to support them both.
On my server I only use roundcube.
This is how to do it as you want:
1. Create a folder /var/www/webmail
2. Install roundcube in this directory (it's really simple, just follow the install instructions)
3. To be able to access it from every vHost, create a file /etc/apache2/conf.d/roundcube.conf with this content:
Alias /webmail /var/www/webmail
php_admin_value open_basedir /var/www/webmail
Allow from all
4. /etc/init.d/apache2 reload
Now the webmailer should be accessible as /webmail from all domains on your server.