var polygons = ee.FeatureCollection('xxxxx');

var bufferPoly100 = function(feature) {
  return feature.buffer(100);
};

var bufferPoly200 = function(feature) {
  return feature.buffer(200);
};

var bufferedPolys100 = polygons.map(bufferPoly100);
var bufferedPolys200 = polygons.map(bufferPoly200);