Just a quick heads up that this guide is not a proper tutorial for the New Game+ categories, but an explanation behind how you can set up the save files to do so.
SAVING THE FILE: In Mega Man Xtreme 2, once you defeat the final boss in X Mission or Zero Mission (Berkana and Gareth, respectively), at the end of the credits, Iris congratulates you. During this, she says you can save your progress and load it again and start the other character's campaign. Afterward, you'll get sent to the save screen and choose one of the three slots to create that save.
After exiting, you return to the title screen. Upon choosing that file that you created, after selecting "YES," you'll immediately begin Zero Mission (if you beat X Mission) or X Mission (if you finished Zero Mission). Your character has the four weapons from the Mavericks the other character would typically fight, plus all DNA Souls you collected up to that point.
PREPARING FOR NG+: There are only a couple of things you'll want to do before creating that special save. The first is that your options configuration from the moment you beat Berkana or Gareth will return to what it was once you load the file. What that means is if you were doing a non-NG+ run and, say, had auto-charge turned off but had it on when you beat Berkana as X, it will switch to that when you start up that NG+ Zero Mission file.
Second, you'll want to collect as many DNA Souls as you can before you create the save. That way, you'll have all the parts in Iris's shop unlocked by the time you beat Skullhead in the intro stage. While bosses give you 200 Souls (sometimes 100 if the game decides to be rude) for beating them, you'll only have 1,400 by the time you defeat Isaz and Sowilo, which is the boss before Berkana or Gareth's level.
As such, what you'll want to do is find a good spot to grind DNA Souls from enemies. The process might take quite a while to do, yet I assure you it'll be worth it to give you all the parts you want at your disposal as soon as possible. -For X Mission, I'd recommend using Flame Mammoth, especially at the conveyor belt sections. Otherwise, Volt Catfish with the enemies around the first horizontal and vertical halls. -For Zero Mission, I'd recommend Blast Hornet. There are plenty of baddies to be found, plus the mid-boss has an endless spawn of bee enemies coming at you. Even better is that the bees can still give you drops when you beat the boss, but try farming for a while first.
FINAL NOTES: Before tackling Berkana or Gareth, I'd suggest saving your progress in case something were to happen. Granted that the game's auto-save would likely have your back if that were the case. Otherwise, keep in mind that when you beat a New Game+ file of X or Zero, the game sends you back to the title screen after talking to Iris. Therefore, you can only save for NG+ from the "start from scratch" playthrough with the other character.
Most importantly, be sure that BEFORE you beat Berkana or Gareth, your options get set to how you want them to be at the start of your NG+ run. Whenever you load a completed X Mission or Zero Mission file, leading into the next playthrough, the game reverts to that particular options setup regardless of whether you changed it at the title screen or not. The first time I created my first NG+ file for Zero Mission, I had no idea about this until later on.
Happy hunting, and have I hope this helps!
