You might want to run more than one Emacs with more than one Gnus at the same time. If you are using different .newsrc files (e.g., if you are using the two different Gnusae to read from two different servers), that is no problem whatsoever. You just do it.
The problem appears when you want to run two Gnusae that use the same .newsrc file.
To work around that problem some, we here at the Think-Tank at the Gnus Towers have come up with a new concept: Parents and children.
Anyway, you start one Gnus up the normal way with M-x gnus (or however you do it). Each subsequent child Gnusae should be started with M-x gnus-child. These children won’t save normal .newsrc files, but instead save child files that contain information only on what groups have been read in the child session. When a parent Gnus starts, it will read (and delete) these child files, incorporating all information from them. (The child files will be read in the sequence they were created, so the latest changes will have precedence.)
Information from the child files has, of course, precedence over the information in the normal (i.e., parent) .newsrc file.
If the .newsrc* files have not been saved in the parent when the child starts, you may be prompted as to whether to read an auto-save file. If you answer “yes”, the unsaved changes to the parent will be incorporated into the child. If you answer “no”, the child may see some messages as unread that have been read in the parent.