In Page1 You Should send the variables to page2,by using the code like
in button1: response.redirect("page2.aspx?value=1");
in button2:response.redirect("page2.aspx?value=2");
in page2 u should take the value in page load
int i=convert.int16(request["value"].tostring());
if(i==1)
{
tabcontainer1.activetabindex=0;
}
else if(i==2)
{
tabcontainer1.activetabindex=1;
}