courtesy runner for the DP????

[slowballbaker]

Need help on this one umpires.....

Starting Line up card...

1 Bob 4
2 Carl 3
3 David DP
4 Earl 5
5 Frank 6
6 Greg 2
7 Hank 7
8 Joe 8
9 Sam 9
10 Tim 1 Flex

At the start of the game..Tim is not the pitcher but David is. When David bats and reaches base can you use a courtesy
runner for David other than the flex?? David continues to bat and pitch until the 3rd inning then Tim enters the game
as the pitcher...but in the bottom of 4th David bats again and Tim is still pitching in the top of the 5th. In the 6th David bats again and Tim is still on the mound the 6 and 7th innings...
This can't be right when is David an illegal sub..or when is Tim and illegal pitcher???? Thanks Sue