RMAN duplex backup. how many channels?

HI,
 We have finally restored and recovered from a near death experience with multiple disk crashes and tape failures. Our env is Solaris (SunOS5.8) and ORA 8.1.7.0.0
 The recovery was incomplete-cancelled and subsequently opened with resetlogs. So I'm thinking it might be backup time!  I had no experience with rman b4 this episode. I am scouring manuals to determin the appropriate rman command to do a duplexed cold backup Our backups are normally hot using rman. we have 2 tape drives attached directly to the box and normally allocate 2 channels, 4 files per set as follows:
 run {
allocate channel t1 type 'SBT_TAPE' parms
'ENV=(NSR_DATA_VOLUME_POOL=GCLCDWP)';

 allocate channel t2 type 'SBT_TAPE' parms
'ENV=(NSR_DATA_VOLUME_POOL=GCLCDWP)';

 backup database
filesperset 4
include current controlfile
archivelog all delete input
format 'FULL_DW_%d_%u';
 release channel t1;
release channel t2;
}
  For this backup I want 2 copies so I have discovered 'set duplex = 2'. If I use this cmd in our env with only 2 drives, how many channels should I allocate? I can't find anything in TFM to clarify but then again I am now sleep deprived.
 Also, I am unclear of the benefit of setting filesperset to values >1. I'd love to hear your thoughts.
 Could the resident experts kindly review this run cmd and comment appropriately?
 reset database # this is required due to the resetlogs  run {
set duplex = 2;
 allocate channel t1 type 'SBT_TAPE' parms
'ENV=(NSR_DATA_VOLUME_POOL=GCLCDWP)';

  backup database
filesperset 1
include current controlfile
format 'FULL_DW_%d_RESETLOGS_%u';
 relese channel t1;
}
 Thanks
Russell

--
http://www.freelists.org/webpage/oracle-l