The team in the lead after 7 innings in the first game of a doubleheader should be able to declare at that time whether they would like to bank runs for game 2, at effectively 50 cents on the dollar. In the Tigers' case today, with an 8-0 lead, they would have been comfortable knocking 4 runs off their tally (re-setting the game to 4-0 after 7) because that lead would still be safe enough for the game 1 win yet they'd now start game 2 up 2-0. If they really wanted to gamble and had enough confidence in their bullpen, they could have shaved off 6 runs, leaving them with a 2-run game 1 lead through 7 and a 3-run game 2 lead to start that game.
Or for either team, starting in the 8th inning of game 1 you should get to choose whether any run you score in the 8th or 9th is applied to game 1 or applied to game 2.