main.go

package main

import "net/http"

func main() {
	http.Handle("/", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.Header().Add("Set-Cookie", (&http.Cookie{
			Name:   "TestCookie",
			Value:  "domain 1",
			Path:   "/",
			Domain: "domain1.com",
		}).String())
		w.Header().Add("Set-Cookie", (&http.Cookie{
			Name:   "TestCookie",
			Value:  "domain 2",
			Path:   "/",
			Domain: "domain2.com",
		}).String())
	}))
	http.ListenAndServe(":8051", nil)
}