I have been facing the same issue, but on the configuration of the second company chart of accounts, what I did was just select the two companies, then select the COA package for my new company, and save, it works.

Working through the same.  Not specifically "easy" so far.
If "record rules" are changed, as noted then, all companies see all the leads,sales orders what have you.  Not the goal.

I found that if I start with a "lead" and have company assigned "show blank" to start, then assign it to the appropriat company.  From there, important to switch to that company, take the lead, created opportunity,  quotation etc.

It would be nice to simply throw quotations/orders over to the second company but that creates all sorts of errors.  Best to start at the lead/opportunity and go from there.

This usually occurs when you have opened a record that belongs to company A (for instance) perhaps for update or something and you try to switch to another company. To avoid this error this error, the record in view has to be closed before before switching to company B.

Hello, 

be carefull when you add a new company, Odoo add a corresponding res.partner be carefull to put nothing in the company field for this one or at least the same as that new company.