I have 2 x 27" monitors. One lamp is perfectly adequate for the brightness.

One lamp with 2 screens is also enough for me

It depends on what your goal is and how the screens are arranged.
I mainly use the Halo as ambient light, i.e. to protect my eyes by illuminating the wall behind the monitor. Then occasionally to illuminate my desk (e.g. scanning documents with my mobile).
One lamp on the main screen is enough for me.