Well, ideally you do both at the same time 😉
But personally, I’ve done it each way.
In my first start-up, I identified the opportunity (implantable batteries made from nanomaterials), and then talked my co-founder into joining me. The gap was only a few weeks, but I validated the idea partially myself, and then she and I further validated it. She was clearly the best one there was on Planet Earth — for this particular idea. She’d done a huge amount of the initial work on the technology.
The second time, my co-founder and I wanted to work together, and spent 6 months on different ideas before we aligned on the one we wanted to do. Then we sort of 🙂 validated it.
Both work. You need both. In the end, they both have to come together at the same time. It’s just where you start.