@Mahesh-Dahampath Zmodeler3.1.2 (sure you mistake is to use pirate version) is deprecated and is too poor handling high poly models. Buy the license, update to latest patch and will to run fine specially in 64 bits version if your system supports it. Other thing is, split the model in 35000 polys fragments at maximum polycount and don't merge them NEVER to avoid this issue.
This is possible. Split-screen coop mod.Same-screen coop mod. The split-screen version will reduce performance by at least half, maybe more, and may not work correctly (check comments). Both versions don't have good controls for the second player due to how the mod controls the player internally.