From my experience and from many successful technical founders, or co-founders, you're either not qualified or just qualified for it. Even if you're advanced in specific areas of programming/computing, being a technical co-founder means doing everything related to the product (I'm guessing it's software). Deployment, database, web app, version control, services, server ops, etc... is a lot of stuff to learn and become proficient with it.
Don't be worried about the sheer overhead of skills or knowledge required. Startups is all about learning, maybe just as much as executing.
Yep, I totally agree. From my (a technical co-founder) point of view - the JFDI approach is a master factor here. If you don't know how to do something - that's good! Take the second step and get to know how you can do it, then double check how to do it right and the just do it. Plus: take the responsibility for what you've done.
Before I started building my first web app in Rails, I didn't even know how to write Rails! I had only previously been a front-end developer, but I can say I learned a TON. Am I a Ruby on Rails guru? Nope, but I know how to traverse the stack with the knowledge I do have and get as much done as possible.
Don't be worried about the sheer overhead of skills or knowledge required. Startups is all about learning, maybe just as much as executing.