1) You need to trust your cofounder(s) entirely. If you don't have trust, you will have fear, and that will lead to resentment and ultimately a breakdown of the relationship.
Doing work as a technical resource looks very different from doing work as a business resource. Technical challenges have demonstrable progress nearly every day, business challenges take weeks, months, dozens of emails, etc to show progress.
Do not confuse "business" with "easy" or "less time". My job is almost entirely administrative, despite doing coding and design a dozen hours a week. Being an admin is perhaps easier programming all day, but it is not trivial. It takes a special type of person to deal with the business wins and losses, give your cofounder some credit.
2) If you're technical, you will do a lot of the immediately hard work. The business problems, as stated above, are long-tail. They take time to show progress, and you have to be patient. I spend most of my days emailing, making phone calls, creating pipelines, financial modeling, and communicating with my employees (coworkers!) about what I am doing has seemed to satiate their curiosity and demonstrate to them that I am working my ass off to make this a great place to work and a great business.
I think you need to work on communicating with your cofounder(s). They should be able to talk about what they are doing every day, loop you into their progress, and demonstrate their value implicitly.
If you don't trust them though, this will never work.