Is multi-licensing a good practice in order to facilitate reuse of copyleft work?
Multi-licensing is most often see as a business concept, and was generally seen where companies would offer code over a strong copyleft license (such as the GPL) and offer their own, proprietary license. I haven't really seen however, much of a use case of offering a work over multiple open source licenses, unless the licenses themselves are incompatible and you wish to make the work more accessible to both parties of licenses.
It's not that common, and you should keep the licensing part concise. Only use it if you have to.
What are the conventions/requirements for the legal notice about copyright and multi-licensing?
Generally, assert your copyright, and make clear your intention to offer the work over multiple licenses. This should be fine.
Copyright (c) 2016 Zizouz212.
This work is available under both the [1st license] and [2nd license]. You may choose a license in which to follow the terms and conditions at your own discretion.
Make sure that you also include a copy of the licenses that you are offering in there as well.