You can set the tab by setting the
MainHUD.Achievements.TabPane.ActiveTab to either 0 or 1.
You can also do this:
/show_window MainHUD.Achievements.TabPages.Echoes
/hide_window MainHUD.Achievements.TabPages.Kos
but while this will show the correct pages, it will not switch the tab itself for you (but since you won't need the tab you could just /hide_window it (or remove it if you want to mod the achievements window).