Yes you can set that up in QBO where one log in gets you access to all QBO subscriptions, I know how to do it when creating the second and subsequent company files, but not how after the fact, suggest you call intuit.

Each company should have its own bank and CC accounts, using the same one is called commingling.

No do not upload the bank at all in your case, you can really mess things up when there are entries that do not belong to the company.  make your entries and do a manual recon, it takes less than 5 minutes for a manual recon