Possible? OpenVPN server requiring both certificate- AND password-based login (via Tomato router firmware)

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http://serverfault.com – I've been using Shibby's build of Tomato (64k NVRAM version) on my Asus N66U router in order to run an OpenVPN server. I'm curious whether it's possible to setup this OpenVPN server to require both a certificate AND a username/password before a user is allowed access. I noticed there's a "challenge password" entry when filling out the certificate details, but everyone says to leave it blank "or else"; I have no idea why, and I can't find an explanation. In addition, I've Google'd this issue a bunch and have noticed people talking about a PAM module for OpenVPN in order to authenticate via us (HowTos)