how to show value in a textbox of .ascx say A from .ascx say B

You have to Create Event Handler for User control like this

User control code behind

public event EventHandler SaveReport_Click;

protected void BtnSaveReport_Click(object sender, EventArgs e)
       {
           SaveReport_Click(sender, e);
           getReportName = TxtReportName.Text;          

       }

UserControl.ascx

<asp:Button ID="BtnSaveReport" runat="server" Text="Save Report" Width="95px"
                                                   OnClick="BtnSaveReport_Click"/>

base on usercontrol button click fire the normal event to call button click.
Previours we created event handler right.. And we create get and set methord to assign the value for that variable. Call the Global event.

private string getReportName;

       public string GetReportName
       {
           get { return getReportName; }
           set { getReportName = value; }
       }

aspx Page.

After implement the user controls, based up usercontrols id we can get that global event in our aspx page.

<uc1:reportdownload id="UCISaveReport"  runat="server" xmlns:uc1="#unknown" />

and we create one more event handle to call the methord and we get that value.

UCISaveReport.SaveReport_Click += new EventHandler(BtnSaveReport_Click);

aspx.cs code behind

protected void BtnSaveReport_Click(object sender, EventArgs e)
        {
            SaveReport();
        }