You could probably do this with 4 pools. Each drive would contain 
volumes in two pools, one for full backups and one for incremental 
backups. For each client there would be two jobs, one backing up to 
drive 1 using pools "full1" and "incremental1" and another backing up to 
drive 2 using pools "full2" and "incremental2". The two jobs for each 
client would use two different schedules so that the jobs run on 
opposite weeks. A schedule for odd weeks of the month might use 
something like Run = Level=Incremental 1st,3rd,5th sun-sat at 21:00. 
Only schedule the daily incrementals, and then run the yearly full 
backups manually.
  

A year sounds like a long time between full backups. With 180 odd 
incrementals for each client you'll likely run out of disk space on a 
USB drive and defeat the purpose. With differential backups run every 
now and then, you could delete the no longer needed incrementals and 
consolidate space.