Hi BigPaws I know what you mean - it isn't easy to get that "benign neglect" of being part of a crowd when you're an only child - hard to disappear into the melee. I have found thinking about "serious talks" and "discipline" as a whole helps - is it true discipline or are you just wanting something different from your child and you win because you're the adult? I have found Playful Parenting really interesting to read, just in terms of thinking about behaviour
If it's serious enough that you'd both be dealing with it if you had more than one child, then you both deal with it - I figure if we had (say) a 5yo as well as DD I would do everything to avoid that being a distraction if we needed a Serious Talk. But for more minor things, especially more spur of the moment it came be a bit unfair on a child to have the eyes of both parents on her, no distractions, so sometimes I just walk away. Or just shut up, or give DH a look and hope he walks away, if I think I'm better placed to deal with the situation.
I'm sure someone more experienced will turn up soon, but for now, there was a big gap between me and my brother and both my parents spoke to me often at the same time in 'talks' kind of thing, I saw this as totally normal, after all, they were the two adults in the family and me and my brother the children, after he came along I never felt their talks to get 'diluted' or found any support in the fact that he was there when I was being told off, etc. I think it's better to have that adult united front rather than being able to 'play' the parents.
DD is getting a bit unruly - she's 10 so probably to be expected! DH and I often have to tell her off or talk to her about her behaviour but my interfering DM says we shouldn't both say things to her as it's like we're ganging up on her because there are 2 of us and only 1 of her. I think that's a bit ridiculous but at the same time I don't want her to feel unfairly picked on. How does it work for other people?