Ok lets put this another way.
mario brothers: this is a 2 player game, but when its the second players time to play you hand off the controller to the second guy. ( at least I think that how it was done )
In this case you both use player one. Both of you would set up on player one and net play would more then likely block the other guy until its his turn to play again.
Contra: this is a 2 player game that allow co-op. You both play on the screen at once.
Some emulators set players to who connects first or maybe can be specified. Both clients are set up as player one but the server figures out how is going to control player 2.
now you said "Is there a way for 2 players to play a single player game with Netplay?" So based on how I explained it above that would be like saying can I play mario brothers in one player mode ( mario only ) and have two controllers operate him. Like example one without the blocking. Could it be done, yes but you need to add this as a feature in netplay and no one would do that as its not practical. So to pull this off you need to modify the net play code.