Both of Orbs are Ancient Orbs. It's called "Light of Orb" and "Darkness of Orb" because two of orbs able to awakens XML and T.XML's true forms. XML's true form require Light of Orb to awaken it. T.XML's true form require Darkness of Orb to awaken it.

But, if one of legends lost their hope, one of ancient orbs no longer works on one of legends. Ancient orbs need works to Legends have hopes...

Their true forms can summons strong weathers because XML's true light form able to summon Extremely Harsh Sunlight and T.XML's true dark form able to summon Thunderstorm.

XML knows Sunny Day move, if XML awaken his own true form, his Sunny Day move will change.

T.XML knows Rain Dance move, if T.XML awaken his own true form, his Rain Dance move will change.