Each player can contribute to the project with non-negative effort.
Player 1's utility is $u_1=e_1(1+e_2-s\cdot e_1)$ where $s\in [0,1]$.
Player 2's utility is $u_2=e_2(1+e_1-e_2)$
For case 1, each player contributes to the project simultaneously.
For case 2, player 1 moves first then play 2 moves next
I cannot find a Nash equilibria (SPNE for case 2) if $s$ is less than $1/4$.
If I draw best a response diagram, the two best response functions do not intersect. But I don't think that $e_1=0$, $e_2=0$ is a Nash equilibrium, since they have would have an incentive to exert more than $0$ effort.
Can $e_1=\infty$, $e_2=\infty$ be a nash equilibrium?