Lets assume a Technical Co-Founder's time is worth $150/hr. A three month PoC built suggests a $70k contribution by the Technical Co-Founder -- upfront -- on the promise that the Business Co-Founder can actually sell it. I'd want some hard reassurances my $70k of contribution can actually be monetized if i'm giving up 50% or more of the equity on trust. If everything is on trust, why not keep all the equity for the Technical Co-Founder?

Note, for most successful Technical Co-Founder, $150/hr is a very low rate.