Firstly you will have to record the change your second webpage has done. You can also set a flag indicating the change.
In your first web page, you will have to poll the web server to check for any changes in the second page. Basically just check the flag. If there are changes, get the changed values (JSON name-value pair would be great) and update your second page.
That's a very naive implementation I told you considering your question. For more complex scenarios, I would recommend taking a look at
ASP.NET SignalR[
^].
If you also want to know how Facebook, Gmail etc do it. Read this post -
How does facebook, gmail send the real time notification?[
^]
Hope that gets you started on a right path!