You can get the index of currentPage in method OnCurrentPageChanged And if the index equals 1(second page) , use the messagecenter to send message to the page.Refer the following code .
in Tabbed Page
protected override void OnCurrentPageChanged()
{
base.OnCurrentPageChanged();
int index = Children.IndexOf(CurrentPage);
if (index == 1)
{
MessagingCenter.Send
in the second page .Move the code that load data from onAppearing to the constructor
public MyPage1()
{
//...
MessagingCenter.Subscribe(this, "click_second_tab", (obj) =>
{
//load your data here
Console.WriteLine("11111");
});
}