var S2 = ee.ImageCollection('COPERNICUS/S2_SR')

var L8 = ee.ImageCollection('LANDSAT/LC08/C01/T1_SR')

var resample = function (image){
  return image.resample('bilinear').reproject({
    crs: L8.first().projection().crs(),
    scale: 20
})}

var L8 = L8.map(resample)

var S2 = S2.map(resample)

var collection = L8.merge(S2)
Map.addLayer(collection)