I have a short repositioning flight that I booked two tickets from different airlines just in case one flight is delay or cancel, then I take the next one.

one is United and one is Delta. They are one hour apart from each other.

I do not plan on taking the delta flight if the United flight is on time. So the questions are:

1. Do I need still need to check in and get delta boarding pass? Or can I just do that as soon as I know United flight is going to be delayed while at the airport terminal?

2. Once I board United, everything goes well with United, can I cancel the Delta flight at that time or no (as too late) to get the ecredit? This is not basic economy. When can I cancel delta flight?