I like the idea of what you are suggesting here. I have worked in places where we have reviewed each others testing to differing levels, whether this be:
“Send me a link to your cases and I’ll review them” - this is usually not the most effective if there is no conversation, but may find some glaring issues
“Lets sit down and walk through your thinking and I’ll help you identify gaps” - will help find more detailed gaps but still isn’t perfect.
“Lets draw a mind map of what ‘we’ need to test and work through whats needed together”
I’ve always pushed for (where possible) number 3, because it’s a collaborative process and removes some of the cognitive biases that come in from being the only testing designing and running tests.
As far as raising defects etc, i think the best way would be to design a boilerplate template of the information needed in the defect, then raise one or two and check them with another member of the team before raising.
Using activities like Riskstorming can also enhance collaboration on testing ideas across a team too and build the confidence of the business in the testing being done.
Sorry, I know there is a lot there, but in my experiences, teams work more effectively when they are naturally reviewing each others work