2K Games have released some information on the multiplayer features of Serious Sam II:
"The multiplayer features in Serious Sam II will allow for full co-operative gameplay on both PC and the Xbox video game and entertainment system from Microsoft. The PC version will allow up to sixteen gamers to fight their way through the complete game via LAN and online, with the Xbox version allowing up to four players via Xbox Live or system link.
Serious Sam II features more than 40 levels spread across seven different worlds, each with entirely different environments. Gamers will wield nearly 20 different weapons and use over 10 different vehicles and turrets against hundreds of enemies, some of which are minute, while others dwarf the player entirely. All of the same gameplay content is available in the multiplayer portion of the game, with minor additions to the maps to make the game more enjoyable for multiple players."