Angelo Alberti can be used only with The Geological Factor expansion, He can avoid the extra or replacement costs illustrated on the left part of the Geological tiles, at the cost of 1 Engineer. You can activate this effect twice in the same round by placing the extra Engineer on the XO action space (you still have to place Engineers during the main action on your turn). If, during your turn, you decide to copy this power with Mahiri Sekibo you can avoid the extra or replacement costs of a Geological tile. You don't need to place other Engineers than the one you use to activate Mahiri Sekibo.