This is a good little exercise combination that can either be used as 1) a compound set (2 exercises back to back on the same muscle group)
In this example it will require some trial and error. Depending on your goals, pick a weight for the incline bench and then pick a slightly lighter than normal weight for the flat bench.
Perform 1 set of the incline bench and either rest for a few seconds while adjusting the bench or go straight into the next set which would be the flat bench press.
2) a mechanical drop set (completing an exercise to failure then performing an "easier" similar set to near failure or temporary failure)
In this example most people can handle more weight for a flat bench than for an incline bench. So you start with the more difficult incline bench and go to temporary failure.
Then drop to the mechanically easier flat bench using the same weight and go to temporary failure.
Good luck and expect some soreness.
Incline Chest Press
Flat Chest Press
This is my, mostly, Personal Trainer musings and information which I hope you'll find helpful!