AFAIK There is no such shortcut.

What you can do is use the sorting office for this pupose.
Create incoming filters that move messages from certain contacts (or groups of contacts) to their own folder.
Create matching outgoing filters that filter your messages to the matching folders based on contacts too.
Now tell TB to view threads in those folders base on headers (or when you've got contacts with really sad mail clients, view threads on 'references and subject')
This way you don't have to start looking for replies or original messages, but you've got them all as soon as you've found one message from the whole thread.

This is the way I sort all my mail and it's working perfectly.