I'm not yet a boost log expert but,
I think what you need to do is to add filters to your sinks so that they only take 
the log record that they are supposed to write.
Basically, boost log is built as a hub:

n log-record ->  1 core -> n sinks

So if you prefer something like that:

 log-record A ->  core -> sinks A
 log-record B ->  core -> sinks B
 log-record C ->  core -> sinks C

Then it's not directly supported at the moment (Andrey correct me if I'm wrong).
So you have to do 2 things:
 - add the information of which kind of log is a log-record
 - add filters to each sinks so that they take only log-records of a specific kind

I'm abstract on purpose because there are several ways to define kinds of logs (basically any comparable data will do).