Perhaps:

    pg = {Polygon[{{0, 0}, {1, 0}, {0, 1}}], 
       Polygon[{{1, 0}, {0, 1}, {1, 1}}]};
    ru = RegionUnion @@ pg;
    RegionPlot[ru, PlotStyle -> Yellow, Frame -> False

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/Yhddm.png