There is no 'multiengine instrument rating' under FAA. If you have the ATP ME (land or sea), you have an airplane category instrument rating that would apply to your SE ratings (even for private or commercial privleges).
The rating doesn't expire
per se, but it can go out of currency.
Electronic Code of Federal Regulations:
Scroll to 61.57 (c). Basically you can stay current by shooting six approaches plus some holding, and intercepting / tracking some type of enroute course. You can also (and if your currency has really lapsed, you may
have to) take an instrument proficiency check from a CFII (or an examiner, or ...). The details can get your head spinning, but this post hits the highlights, I hope.
Oh, and it's a
category rating, as I mentioned. So you can do the approaches, etc, in a single-engine
airplane (as well as a simulator, etc).