Redmine

I have two Redmine online,
Both are using the same version: 3.0.4 with postgresql 9.2

Redmine A => with all the stuff and users, historical
Redmine B => with only some new projects, and only a few users

I now have to fusion those 2 redmine, and there are several problems:
1) id of projects which will be in conflict
2) some users from A are already created in B

I have no idea how make them as one, without losing data or users