I the future, we might want to run both RapidInformer and RapidInsight on the same server and to the same audience.
1) Will it then be possible to manage Users and Group in a unified way, i.e. avoid creating twice the same users and groups ?
2) Beyond that, would it be possible to have the RInformer user interface as an additional RapidInsight tab ?

1) It is currently possible
1) It is currently possible to run RapidInformer and RapidInsight with a common user/group system. In fact, the RapidInformer and RapidInsight servers running on this site work in this way.
The users are created for both systems during registration, but there is a single place for authorization, groups, etc.
2) With the next release it will be possible to implement custom/customized user interfaces for RapidInformer, hence it will be possible to include the functionality as a tab in RapidInsight as well. RapidInformer (as well as RapidInsight) server functionality is all exposed as web (XML over HTTP) API, therefore it is already possible to develop custom UIs. The new functionality will make this easier and provide examples of how it can be done.