Trust is important.
But beyond that, a good partner might be one where you can first successfully try to come up with an arrangement that you would each be delighted with at the 50/50 point.
Without any adversarial attitude, what can really work is that kind of baseline, which can be negotiated away from, but also returned to without disdain.
There has to be a sense that each person at the baseline would generously actually be putting in 60% for the foreseeable future and love it because it was still an equal partnership and you were getting 120% accomplished consistently. Ideally you want to build consensus not by trying to limit contributions to keep from putting in too much, nor valuing other contributions for less than they are. When it is 50/50 it's more likely to be harmonious when both partners step up to the plate simultaneously and try to put in more than their fair share when needed, and see the other partner as an over-contributor just like themself.
Like what if somebody thinks they're hot and can raise a million within one year, or less if you have something tech ready to deploy? Or they say they can sell your product through the roof. You could make that a contingency they would be proud to achieve or they would have to understandably not be up to their share. If you came to a tentative agreement like that, that you were both very happy with at 50/50, all you would be doing next is planning to "start your engines" soon.
If the potential partner comes back a couple days later with a $4 million commitment within a month by selling some property or something, and it's for real, you will probably re-negotiate. The contingency would be over in a month then, and you would be flying. This wouldn't be too bad if it was non-adversarial from the get-go. You could end up at something way different that's quite fair anyway.
Remember each engineer is worth a million in the right situation, now with their $4 million it's a 4:1 imbalance, so 80/20 might be easy to migrate toward then so you can still go forward.