By CTO co-founders, I assume you are talking about technical co-founders. I think I can make a few illuminating points about that, since I am a technical co-founder.

I have a pretty solid reputation in my tiny startup community, and I have been approached quite a few times by people who wanted me to partner with them. Each time I have met with these people, I look for the same set of things before making a decision, which is almost always a no (unfortunately).

I do not care about the idea

I have ranted extensively about this before:

Tahmid's answer to App developers: I need someone who would like to be my partner (50-50). I have an idea of an app but can't code it. Is anyone interested?

I will say it again, I think an idea by itself has no merits. If my potential co-founder is too protective of the idea, or asks me to sign an NDA, I immediately lose interest. I especially dislike people who ask for equity on the sole basis of providing the idea for the business.

You have to bring something to the table

This ties back to my previous point. You have to bring something to the table, preferably in the form of a rough business plan, sales strategy, marketing decks, etc. Previous experience in these fields is even better.

It takes me 3–4 months to build a solid product, I will not accept a partnership offer unless I know for a fact that my partner is working his/her ass off during this time period, to make sure that the product gets traction. A small monetary investment into the company is also a good sign for me, because it indicates my partner has some skin in the game.

If you have none of those, then at least have some positive results:

• Sign ups on a pre launch page, something that can be made easily using a service.
• Positive affirmations from potential customers.

Once I was approached by 3 people to partner with them. Really nice people, but I could almost smell their inexperience. They told me we could get to $200,000 MRR (Monthly Recurring Revenue) within 5 months, roughly putting us at 10,000 customers, because the idea was so damn good. I am not even joking. I told them I would build them the product, but only if they could get positive affirmations from just 10 potential customers. They came up with 0. The partnership did not happen obviously, because they brought nothing to the table.

So yes, if you want a tech co-founder, care less about the idea and bring some complementary skills to the table, these usually being sales, and marketing. Personally, I do not care about the skill of designing mockups at all in my co-founder, and I am pretty sure I am not the only one.