Can I cross-post an entry in multiple journals or communities?
LiveJournal does not offer an automatic cross-posting feature from its web interface. However, you do have several options for achieving a similar effect.
First, you can simply compose and post a separate new entry in each of the desired journals or communities. Writing the entry first in a text editor may make it easier to cut and paste the entry each time.
A second option is to post the full entry in only one place and then link directly to that entry from the other journals or communities. http://www.livejournal.com/support/faqbrowse.bml?faqid=67 contains information on creating a link. When using this method, you may want to disable comments on all but the original entry. This ensures that all comments are contained on only the original entry. Information on disabling comments can be found here: http://www.livejournal.com/support/faqbrowse.bml?faqid=22.
Finally, some downloadable clients do allow cross-posting or have features that make it easier, such as the ability to save a draft copy of an entry. See http://www.livejournal.com/faqbrowse.bml?faqid=158 for information on downloadable clients.
Please note, some communities have strict guidelines regarding cross-posting. Ensure you check the relevant community info pages for any rules/guidelines prior to cross-posting. Some may specifically prohibit cross-posting, others may simply just require that you use an lj-cut (http://www.livejournal.com/support/faqbrowse.bml?faqid=75).
Editor's note: Something like "If you would like to cross-post the entry in other personal journals of yours, [link to following proposal] explains how to do so." should probably be stuck at the bottom of this, assuming both of these get added. My wording sucks.
Can I link my journals together? How do I update another journal without logging out?
While there is no way to link two or more journals together under a single username, there are several options for updating other journals without logging out of the current journal.
First, if you update via the web interface (http://www.livejournal.com/update.bml?mode=full), you can use the "To post as another user, click here" link to the left of the date and time fields to specify a different journal to post in. This will allow you to specify an alternate username and password as the destination for your entry, while you remain logged in to the current account. This alternate login page is directly accessible at http://www.livejournal.com/update.bml?mode=full&altlogin=1.
A second option is to use a downloadable client that allows you to log in as multiple users simultaneously. However, not all clients support this functionality. You will need to consult each client's documentation to determine whether this feature is available. Information on obtaining a downloadable client is available here: http://www.livejournal.com/support/faqbrowse.bml?faqid=158.
Finally, you have the option of converting a secondary journal to a closed community and then granting only your primary account posting access. This allows you to post directly to the closed community from your primary account. However, please note that a journal being converted to a community must not contain any entries. If you have existing entries in your alternate journal(s), this is probably not the best option. Also, once a journal is converted to a community, it can no longer post entries to itself. Information on creating a community is available here: http://www.livejournal.com/support/faqbrowse.bml?faqid=78.