multitail has several interactive keys of which
b will popup a menu asking you to choose which window to scroll. Use the arrow keys to navigate and press enter to select a window. That window will appear overlaying all the other windows, except for a small border of 2 characters in which you will be able to see any changes. You can scroll this chosen window with the arrow keys, and
q to exit this mode. I don't know of any way to scroll whilst keeping the window arrangement intact.
h to get the popup help on the interactive keys available.
A hack you can do is run
tmux in a small pane, use b to create the scrolling subwindow, then make the pane full size. This does not change the size of the scrolling subwindow, but does make the logs visible in the new space. If you already have a tmux session with one window, then you can experiment from another terminal with the commands:
$ tmux split-window
$ tmux resize-pane -y 10
$ tmux send-keys 'multitail /var/log/maillog /var/log/messages /var/log/httpd/access_log' Enter
$ tmux send-keys Enter b Enter
$ tmux kill-pane -a -t.
These send commands to the session to split the window, reduce the height of the active pane, and run a multitail that will say there is not really enough space. The second
send-keys acknowledges this, and enters
b for the scrolling subwindow. The final caommand kills the other pane and a full-size multitail is seen, with a small scrolling subwindow.