2. After you finish the second you can keep playing. The enemies will be level 47-51 and the bosses will start respawning as well.
After finishing the first playthrough, the second playthrough becomes unlocked, allowing you to complete the game again from the start, only with all of your items and weapons from the first playthrough. The enemies in playthrough 2 are higher levels, and have different names. They also use different styles of guns. Upon the completion of this playthrough (killed the destroyer), all the enemies become level 48-51, have different names, and become generally more difficult to kill. Additionally, almost all of the side quest and main storyline bosses begin to respawn, including Mothrakk, 9-Toes, Skagzilla, Moe and Marley, Sledge and so forth.. only the destroyer doesn't respawn.