w[y1_, y2_] := y1  -  (y1^2)  +  y2  -  (y2^2);

gr1 = Plot3D[w[y1, y2], {y1, 0, 1.5}, {y2, 0, 2.1}];

gr2 = 
      ParametricPlot3D[{t, 1/2 - t, u}, {t, 0, 1/2}, {u, -1.5, 0.5}]

gr3 = 
      ParametricPlot3D[{t, 2 - t, u}, {t, 0, 2}, {u, -1.5, 0.5}]

Show[{gr1, gr2, gr3}, BoxRatios -> {1, 1, 1.5}, 
      ViewPoint -> {1, -0.8, 0.9}]