Hello campersand and Welcome to Seven Forums.
In WLM 2012 "All sent items" I see emails sent from all accounts including my Gmail IMAP account.
I have a Gmail IMAP and Hotmail MAPI account in this WLM setup.
I sent a message from my Gmail IMAP account using Outlook 2010 on a different PC, and that message is listed in WLM 2012 on this PC.
It might help if you specify what programs you use, what email providers, what WLM version, etc.
Have you checked your settings on your Email provider(s) Web site?
Make sure all programs use the same protocol per account such as IMAP.
Hope this helps,