[imp] Get threaded messages from two, not one mailbox

[Kristers Zariņš]

With Horde Imap Client library I'm able to get threaded messages from one
mailbox, but this is useless because I cannot get them from two at the same
time. What I mean is that I can get only one half - those stored in INBOX,
but not the other half - those stored in SENT folder. The code I'm using is
roughly like this:

$Imap_Client_Data_Thread = $Imap_Client_Socket->thread('INBOX');
$Imap_Client_Ids = $Imap_Client_Data_Thread->messageList();
$raw_uids = $Imap_Client_Ids->ids;
$last_id = end($raw_uids); //For example, lets get thread for the last
message in the INBOX
$thread = $Imap_Client_Data_Thread->getThread($last_id);

$thread now contains one half from all messages within the thread - those
in INBOX mailbox. But the other half - withing SENT - is missing.

How do I get the other part?