Greg,
Normally you could use Policy based routing, but the ASA does not support this (yet), if you have a router in front of the ASA (so between the ISPs and your ASA), you could use PBR to split traffic between providers.
check out this post:
https://supportforums.cisco.com/docs/DOC-6069 it contains a discussion around using NAT to engineer what you are trying to achieve.
please rate if usefull.
Dennis
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