1. Product beginning to gain traction relatively easy, not a problem getting some users (B2B). Not much effort put in/no focus on user growth yet.
2. One year+ of highly technical development work done after work/weekends.
3. No marketing/technical docs/customer onboarding/sales etc.
4. Future work is extremely technical, but lots of working with customers will be needed.
5. No VC funding/very little revenue.
So basically all of the "company" is code. How do you bring someone onboard in this circumstance? Seems silly to give them huge equity, but it's very early, early enough its hard to imagine someone good joining for small equity.
Another big question is, does the co-founder bring anything tangible to the table. Do they have an online following, sales/marketing connections, experience building a business.
Really you do want someone who is an equal partner, who's value makes the pie bigger. If you can find such a person, they are totally worth 50% equity (actually slightly less, since you want a third minority owner to act as a tie-breaker.). But the only way to really vet such a person is to see them in action, which requires them to volunteer at first probably.
Maybe you could work out a commission deal for revenue share. That allows you to motivate a partner simply, without selling off equity. (Remember giving away equity also costs money in lawyer fees).