If you book two separate tickets, you will not be connecting at all. Rather, you will be ending one itinerary and starting another. While US is generally pretty good about rebooking in this situation, you come at the bottom of the heap, it may take significant time to reinstate your xPHL ticket which would have been cancelled when you no-showed and, if the IRROPS were caused by something within US' control such as MX, you are on your own for hotels + meals if needed.