For 4 years as an undergrad in Iowa, I did this about monthly: booked a PM flight DSM-MSP and flew standby for an AM one.

When you check in for your original flights, they should print your evening MSP-LAX. Then when you get onto the CMH-MSP nonstop, they'll collect your CMH-DTW and DTW-MSP boarding passes and print one for the nonstop--- but they should let you hang on to your original MSP-LAX. You probably won't even have to talk to an agent in MSP-- just get onto your scheduled flight and you should be fine.

As far as what to do if only 1 seat is open, you shouldn't be obligated to do anything-- and the gate agent should clearly understand if you are a party of 2.

[This message has been edited by Viajero Joven (edited 12-06-2000).]