To expand upon what Glen already notes, if what's being used for the channel hash doesn't change, traffic will use same link. Assuming the backup is running between a pair of hosts, using src-dst IP will always hash to one link. Other hash algorithms (if supported) will have the same result for any single flow. If there were multiple concurrent flows between the hosts, and ports number were also part of the hash, then multiple links might be used.
If concurrent flows with a different hash algorithm isn't an option, as Glen notes there isn't much more you can do with Etherchannel. Other options might include using a higher bandwidth link between the 6509s (e.g. 10 gig) or perhaps splitting the channel into either dual, dual Etherchannels or four individual links and using packet-by-packet routing. (The latter adds the issue of out-of-sequence packets. Shouldn't be a problem across two links between the same two devices, could be a problem with four links.)
