How do co-founders share AI context without losing private stuff?

My co-founder and I have a problem neither of us expected. We each go deep on our own projects and lines of thinking in our own AI accounts. But none of it syncs. My AI knows everything about what I've been working through, theirs knows their side, and neither knows what the other learned.

Over time this has become a real knowledge gap between us. We've tried shared folders and pasting summaries back and forth, but it doesn't keep up.

Feels like there's a genuine opportunity here though. A shared AI brain that a whole small team can read, with private stuff actually staying private.

Anyone solving this well? Do you use Claude Team or ChatGPT Business? Or a shared memory tool like Basic Memory or Supermemory? Does it actually stick?

If someone built this properly—one shared brain everyone's AI could reference—would you pay for it?