It's likely that based on your question, you're not yet ready to find the right person because you haven't defined clearly enough who you're looking for and how they fit into your company.

Mike's answer is great and answers the question most directly. I would add that what you're really looking for is someone that knows you well and knows a potential candidate well enough to feel comfortable making the introduction and endorsement of both parties. As Mike points out, a co-founder is similar to a marriage and one where personal chemistry is the most important test to determine potential for a lasting and meaningful relationship.

But your question detail has me wondering why you're looking for a technical co-founder if - as you say - you're looking for redundancy? If you're strong technically, why not hire a Sr Engineering Manager or Product Manager or even Director of Engineering? All of these would likely be single percentage equity hires vs a double-digit co-founder.

So again, get really clear on what you're looking for (the attributes that add to your existing bench strength), why you're looking for that person (how you see their responsibilities interacting with the responsibilities of others in your org, especially yourself), and what the right fit is going to "feel like" (i.e. culture fit definition).

Once you can articulate these things clearly, it'll be easier to find that perfect candidate.

Happy to talk to you about any of this in more detail in a call.