Pardon my ignorance, but I have a SDC question.

I had a reservation with my SO, and split it to get better EUA chances. I was thinking of trying to take a flight 10 hours after our current flight is scheduled. As a CO Plat, when can I try to SDC to the later flight? Online checkin 24 hours before the original flight? Online checkin 24 hours before the flight I want to take?

Since the res was split, can my SO SDC to the same flight for free?

Thanks for your help! I haven't really flown CO before.