High Sierra (this community), or Mojave (your footer)?
On High Sierra...
⌘T is the new-tab shortcut, not ⌘D.
⌘D gets you two viewing panes into one tab, and into one session.
Panes are handy for looking at different parts of a long output buffer from a single session.
I'm not aware of a way to fork a paned terminal session into two separate tabs.
It is possible to drag a tab off of a combined multiple-tab session to get separate displays, similar to how tabs work in Safari and some other macOS tools.
If you want multiple panes with different sessions, iTerm 2 is a potential choice.
If you want multiple sessions in one pane, launching screen or tmux can be an option.