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

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

or

BoundaryMesh[DiscretizeRegion@ru, MeshCellStyle -> {1 -> {Black, Thick}, 2 ->Yellow}]