You extract wood tar by applying indirect heat (ie to the outside of your retort), the by-product is charcoal, this is normally used to fuel the next batch
So, you're right, two products made in one process
I think you'd need some sort of sump to collect the tar and maybe a 'U' bend sort of thing in the outlet pipe to keep the air out
If all you want is Stockholm Tar then it doesn't matter if air gets into the hot retort and eats your charcoal, you however are planning on doing what I'd do, getting the maximum stuff out for the minimum work in......I'm not lazy, I'm efficent*
*and dyslexic, but thats another story
Shed bushcraft, like the real thing........honest!!