Yes.
Mayday and 7700.
No question about it in a twin.
Why?........because you have lost one of your two engines and at that very moment you probably don't know exactly why and need as much ATS assistance as possible.
If you are one of those that can guarantee that the other will not fail and that you will not have further complications with load shedding, Vacuum supply, performace and so on, then perhaps downgrade it to a Pan but I can't see the point in downgrading when you may have to upgrade again later.
Mayday is simply a tool for obtaining unhindered assistance from the rest of the team i.e. ATS and aerodrome providers.
However, one of the most important things is that you shut everyone else on the frequency up while you get it sorted, gather your thoughts and get the assistance you want.
Regards,
DFC