Regularly, people like you ask us why Coccinella does not support multiple accounts. Or they ask if support could be added. We always have to disappoint them. Coccinella does not support multiple accounts and it is very unlikely Coccinella will ever support multiple accounts.
In order of importance, these are the reasons why:
Coccinella is targeted at stupid people for whom this feature does not add any value. We define stupid people as those who do not yet use XMPP or those who never used instant messaging before.
We also believe that support for multiple accounts destroys value for these stupid users. Did you have ever seen any official proprietary network instant messaging client with this feature? Did you have ever seen an instant messaging client with this feature without related usability issues?
Stupid people want to start with Coccinella without using their brains. A failure in providing a brain-less interface will wipe a new XMPP user away even before he could get started.
We do not target Psi users, Gajim users, Pidgin users, Kopete users, and so forth. The minority in the whole instant messaging user population that really needs support for multiple accounts already has plenty of client choice. Coccinella is not yet another look-a-like; Coccinella is leading the way instead of following.
If people really need support for multiple accounts they can use a server-side XMPP-to-XMPP transport.(*)
Adding and maintaining support for multiple accounts is a challenging task requiring many resources that can better be spend on more sexy features like video conferencing. Or in Mats words:
"If I may take the moderators middle standpoint there is one fact that makes the question simple. Although the jabberlib is built in an OO manner, the rest of the code that interfaces jabberlib isn't structured in a similar way. It is therefore a huge task to restructure essentially all code to keep track of which jabberlib instance to use for each task. Essentially all code, or at least a lot of code, need an extra jlibname argument, and I can tell you it is a lot of work."
(*)After recently trying both J2J and the IM Gateway plugin (for Openfire only), I have to agree that the current state of both solutions is not ideal. The least worse of both is J2J which does not show your contacts of the other account in your list. The Openfire transport is quite useless for the simple reason that it is limited to only 1 predefined server. So, if you have an account on the jabber.se server while the administrator of your server configured the Openfire plugin to use jabber.org, the transport will not work for you. Besides that, the Openfire plugin only works with Openfire. Hopefully, all these issues will disappear in the future. Though mostly focused on proprietary networks, our wishlist for transports lists some points also valid for XMPP transports.