Inside any pane, press
<Prefix>! and tmux will create a new window from your
pane, removing the original pane.
Now, to move the first window into a pane in the second window, press
<Prefix>: to enter Command mode, and type this:
:join-pane -s panes:1
:join-pane -s :1
You can use this technique to move panes around as well. If your first window had two panes, you could specify the source pane like this:
:join-pane -s panes:1.1
The resize-pane command accepts the -Z option for zooming a pane. Best of all,
it’s already mapped to
<Prefix>z, and pressing it again restores the pane to
its original size.