What is more difficult for a Jedi to defend against?

1. An opponent dual wielding individual lightsabers (like Ahsoka)
2. An opponent wielding a double bladed lightsaber (like Maul)

Depends on the Jedi.

Boba Fett.

It should be harder to defend against an opponent wielding two separate lightsabers. Two separate lightsabers can attack simultaneously from completely different vectors. Meanwhile, a double-bladed lightsaber actually reduces attack options (For example, a perfectly vertical overhead strike wouldn't work with a double-bladed lightsaber, you would impale yourself), and as long as you know where one blade is, you know the other blade is facing in the exact opposite direction.

There's no question in my mind that a duel-wielding opponent should realistically be more deadly than an opponent with a double-sided lightsaber.