Architecture is monstrous in the way in which each choice leads to the reduction of possibility. It implies a regime of either / or decisions often claustrophobic, even for the architect. Seemingly, the extension of the Stedelijk Museum presents such choices. The fact that two consecutive directors think differently about the crucial question – new in old / old in old / old in new / new – reveals the sometimes unwelcome 'power' a discredited profession continues to wield: the power to make things impossible. To avoid this dilemma, we have taken the word 'extension' literally; where a separate building inevitably raises the issue of division, we imagine the coexistence of two parts in a form of complete mutual dependency, and to some extent, interchangeability; in this project we are not separating but fabricating Siamese Twins.