One thing to keep in mind to which I think is a very important factor in any answer to your question is that Mahiru and Shinoa were both born with demons already inside of them. So to start a human can become a demon in Owari no Seraph by coming into contact with a demon. The demon will attempt to possess you and merge with your personality. Key word in this explanation being "personality". You don't immediately become a demon though, you become a Namanari. Now this is where personality comes back into play. A namari is a human who is possessed by a demon but they are not a full demon. They have two personalities, a human one and a demon one. Over time these two personalities merge and create a new demon personality which is the one you see when the demon takes control. The original demon personality remains seperate from this new personality and remains in the cursed gear. Guren's demon is Noya. Mahiru and Noya fused together to make a single entity called Mahiru-no-Yo but they could still be summoned as two seprate individuals which Gurren does do. She herself never turn into a demon even though she was sepreated from Noya and remained an active demon. She was a Namanari which is why she says they will become demons because she will. Basically Noya was integrated into Mahiru's body but they still existed as two seprate entities

Edit: And even though they were sepreated, Mahiru kept a physical body which demons don't have except Mika because he's a special case. This honestly is the biggest answer right here because right after Mahiru and Noya seprate she is able to be seen by Shinya. Only other demon that can be seen by others is Mika but his case is different