CONDITIONS
When X1 > 0.5, fX1 | X1>0.5 ( x1 | X1 > 0.5 ) = 2 fX2 | X1 ( x2 ) = 1 / x1
When X1 < 0.5, fX1 | X1<0.5 ( x1 | X1 < 0.5 ) = 2 fX2 | X1 ( x2 ) = 1 / (1 - x1)