If you are on two tickets and the flight from Mississippi to Boston is delayed you're up the proverbial creek without a paddle. There is no reason for US to link two tickets that you bought on the cheap to create an ITIN that they would have charged you more for had you booked it that way. This isn't in their interest.

The only good plan is to plan way more time than necessary for the connection and hope that your 1st flight isn't cancelled completely.

I used to do this all the time to save money and one misconnect when my flight to LHR landed at MAN instead burned up all the money I'd ever saved when I needed to rebook my LHR-FRA connection on a list minute and very expensive fare. I was lucky that they let me keep the return portion of the wasted LHR-FRA, in many cases they'll just cancel the return as well.