In 2-3 player Citadels, can your second character buy a destroyed building using Graveyard?
No. Machiavelli, a.k.a. Citadels, has a purple building called Cemetary/Graveyard. The Warlord or (Condotaire) is not allowed to buy the destroyed building. You don't really have an alter ego, you have two turns during each round. You can only destroy a building during your Warlord turn, and during that turn you are the Warlord and will not be able to use the Graveyard/Cemetary ability.

Graveyard - When the Warlord destroys a district, you may pay one gold to take the destroyed district into your hand. You may not do this if you are the Warlord.
In a two- or three-player game, all players play with two characters each. The game is played normally, except that each player has two turns each round (one turn for each character). Players do not have to separate their gold or their districts between their characters, as the players still only have one city.
Each character has a special ability, also called its power. You may use your character’s
power once during your turn.