Help get this topic noticed by sharing it on Twitter, Facebook, or email.
CHAMP0Fist of all, try Unified Folders.
or these arrows
This preserves the separate folders but creates a composite single folder. You will still see the separate accounts, but the topmost group of folders are a combination of all the corresponding folders in each account. Each can be exploded to show the actual folders.
If this is not to your liking, then in some cases you can arrange for a genuine single folder.
If both are POP accounts you can use the Global Inbox.....you can see the account type here:
Tools|Account Settings|...select the account...|Server Settings→Advanced
Select "Global Inbox (Local Folders Account)"
and also check "Include this server when getting new mail"
Do this again for the second account.
If they are not POP accounts (in which case they'll be IMAP) then you'll have to use the Unified Folders approach.