My co-founder just left our company after 4 years because we basically had to pivot and restart with a new product that fit with COVID (we are in the events space).

Looking back at everything that lead up to him having to leave, I've come up with the one mistake I regret doing:

Splitting up duties.

We were technical founders but we decided one of us would handle the business side and one of us would handle the technical side.

I think this ended up isolating us rather than having us work together as a team. As we grew I hired account managers, sales, etc and he hired devs. We chose to start a company together but never really worked together.

Now 4 years later I'm working in the backend for the first time. I wish I would've spent more time pair coding with him and discussing architecture together. We worked at 3 other companies together and went to university together and I think we missed our opportunity to truly work together on something WE want to build.