rman question configuring channels

oracle 9.2.0.4
for rman
If I have my parallelism set to 2 and have two backup channels configured going to two separate mounts. One is fibre, one is scsi and lets say writes to the scsi mount are half as fast as the fibre. After the backup completes, should I expect the size of the fibre mount to be at least 2x as large as the scsi mount? I ask this because due to space constraints we slapped an old scsi disk array on to our server. I was going to swap one of my rman channels out to it, but it's so slow I don't think I'm going to gain much. Just testing now.

Thanks
Mike

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