I want to merge two shape files in which one is of point geometry type and other is polygon type in order to get a single layer containing both points and polygon in a single map.
Shapefile format doesn't allow multiple geometry types, so you should use another format, like GeoJSON. See also OGR merge shapefiles of different geom type into a single GeoJSON feature collection, How to Merge/Join GeoJSON and JSON? and https://gist.github.com/migurski/3759608
But you can group two layers together if you are trying to save mouse clicks on switching them on and off.
You can buffer the point and save it. Then in QGIS go to Vector -> Data Management Tools -> Merge shapefiles to one. If you want geojson, then you can now convert the merged shapefiles to geojson.