In the first case the term "two party" is describing a defence (consent). Defences may vary from jurisdiction to jurisdiction, so the term might be useful in h

In the first case the term "two party" is describing a defence (consent). Defences may vary from jurisdiction to jurisdiction, so the term might be useful in highlighting those differences. In the second case the term ("assault") is the offence itself. It is not a term that is being used to highlight differences in elements or defences amongst jurisdictions. Being accurate in the first case makes sense, regardless of the audience.