Is it Possible to Save BOTH Jean Grey and Nightcrawler in Mephisto's Realm?

You are wasting your time. You can only save one of them, the other will fall to their death.

IGN's walkthrough states this, as well as other youtube let's plays as they state that you can only save one of them.

Whoever you don't choose to save will end up attacking you during the boss phase and sacrifice themselves in order to save your party.

If you have magneto in your team you save both.

When you choose one X-Man to save, Magneto will say that the cages are still made of metal and not to worry. He stops the other from falling and both Nightcrawler and Jean Grey walk away alive. During the end of the battle of Mephisto, both of them will help you as well.