It doesn't involve murder, but I know of at least one case as a fact.
2 years after I graduated from high school, my home-ec teacher was charged with sexually assautling her teenage foster daughter for a period of over 2 years. The assaults involved my teacher- Peg Groenwold, and her husband Jerry- both from Brainerd Minnesota, performing oral sex or sexual penetration weekly, and other various assaults several times a week.
I remember being utterly shocked, and never having heard of this before. So, that's one case I know of, too.
ETA: the assaults did not happen necessarily while husband and wife BOTH participated. It sounded more like they took their sick turns on the girl. I pray for that young woman (now) a lot.
:behindbar:behindbar