...anything is always possible...it's the time/cost of the path to get there...Is there a way to make Advanced Shipper aware of Better Together?
Some generic advice...
I would see if this discount scheme works with the standard Zen Cart shipping methods first on a clean installation to check the issue is not with Better Together.
Then figure out what variables are beng used to carry this data (Superglobals) and see where/if Advanced Shipper is not dealing with them the same way.
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