This article explains how we can find the controls from content page to content. First we will find the contentplaceholder of Master page. Because all content pages inheriting reference of that contentplaceholder.

in This example we have two content pages. Content1.aspx and Content2.aspx. Now we need to search the label "lblHeading" placed on Content1.aspx, from another content page. named Content2.aspx. Now we will put below code on Content2.aspx to search the label

ContentPlaceHolder contentPage = Page.MasterPage.FindControl("ContentPlaceHolder1") as ContentPlaceHolder;
Label lblHead =(Label)contentPage.FindControl("lblHeading");
Response.Write(lblHead.Text);