2C-mode-line-format

2C-mode-line-format is a variable defined in `two-column.el'.
Its value is

("-%*- %15b --"
 (-3 . "%p")
 "--%[(" mode-name minor-mode-alist "%n" mode-line-process ")%]%-")

Documentation:
Value of `mode-line-format' for a buffer in two-column minor mode.

You can customize this variable.