If you use 1 envelope separate the 2 cases with rubber bands or fasteners: https://www.uscis.gov/forms-filing-tips
Yes, 2 checks; $535 each
Or you can use a credit card or debit card: https://www.uscis.gov/news/news-releases/uscis-expands-credit-card-payment-option-fees
U.S. Department of Homeland Security
Treat each I-130 as it's own case. Each I-130 will need a copy of the required documents.
That is correct.
That thread is incorrect. It's needed later at the NVC stage.
