Read this series around about 2008-2010; thought they were either by Rachel Caine or Kelley Armstrong, but from what I can see I must be mistaken.
From what I can remember of the concept, humans and demons had agreed to allow demons to share the body of humans. Our female lead keeps a strict control over her inner demon, not trusting that it won't try and take full control over her. Her partner (who for some reasons I think is called John, but I may well be misremembering that part) openly shares his body/soul with his demon, and thus whilst he acts in a normal human fashion, it's always the demon part of her partner that has conversations with the female lead.
The hosting of demons within humans was negotiated between the governments of the humans and hell and thus was not a secret agreement, and humans volunteered to be hosts. Cannot remember why our female lead became a host given her will to remain in control of the demon inside her, but on occasion she did allow her demon to take control, under strict conditions.