The only way that I can think of is to create 2 onsite primary pools and
change the management classes before each backup to what you want it to be.
Unfortunately, that means the init[SID].utl file has to be different for
each primary pool and you will have to do some switching of the file right
before each backup.  What I would do is create is an init[SID].utloff and an
init[SID].utlon and copy it to the init[SID].utl right before the BRBACKUP.

Remember, though that the restores do not care about this.  TSM will just go
get the data whereeever it is.  But, it will care about the archive
management class used for BRARCHIVE.  I would not try to do this with it.

I am not an expert in this area, but it is a way to accomplish what you are
trying.  Ultimately, you have to get the backup you want to copy and send
offsite to a different primary storage pool.  So that only it is copied and
sent offsite.