I don’t know of a way to create a rule to <i>move</i> sent messages to other folder. At most you can create a rule to <i>create a copy</i> of the messages in other folder.
Something similar to what you need could be achieved in the following way:
– Create a rule to move a copy of messages sent from your primary account, to some folder.
– Create a rule to move a copy of messages sent from your secondary account, to some other folder.
– Uncheck the option to save copies of sent mails in the ‘sent items’ folder.
The main differences with this approach would be:
-The rule will put the copy of the message in the desired folder, right after you click on the ‘Send’ button (no matter if the send/receive process fails).
-If the rules fail or get disabled for any reason, you won’t have any copy of the messages.