If your league is configured for one starting QB, K, DEF, etc., it isn't possible to start more than one player at that position for a single week.

This applies to all positions on your roster. Each roster position becomes locked five minutes prior to the kickoff of the game involving the player occupying that position.

So, it wouldn't be possible to start one QB on Sunday morning and then substitute another quarterback for the Sunday night or Monday game, again assuming your league uses only one starting QB position.