I work for a tiny 2-man project and we need to save all the time we can, so we don't code review. We actually have an unofficial policy: all code that is directly user-facing (web design for example) is handled by me and all backend stuff is handled by the other guy.

My previous company worked the same way though, and it was a real company with funding and customers. It was another tiny startup (essentially 4 person engineering team) and the work was cleanly divided: one person did the entire iphone/android app (react native), one person did the website, one person did the backend and one person did the analytics. For an established company this would be a nightmare -- we had tons of bugs in production, usually reported by our users. However, having a really clean separation of duties made it extremely efficient in terms of management, because everyone knew that if they wanted something done on the mobile app they just asked the guy who did the whole thing, he knew the codebase thoroughly and could fix or add the change almost immediately, then would work on the next task on his trello board (which almost nobody else even looked at). We essentially had no management (CEO was constantly gone looking for more funding) so this organizational structure was a huge boon, and allowed us to push out huge features in 1-2 weeks. I think that if we had a full time QA person this really could have worked well, and by "well" i mean "we could have done the job of a ~15 person engineering and QA team with only five people."