A Burst Synchro Summon can be conducted when there are two or more monsters whose Level will equal the Burst Synchro Monsters, but they will use a Burst Monster as one of the materials to Synchro Summon. Those monsters can be used as Material Monsters to Summon a Burst Monster whose Level is equal to the combined Levels of the monsters on the field.
Just like Burst Monsters Burst Synchros Material Monsters are not sent to the Graveyard. Instead, they are sent to the player's Deck.