Because of consistency, an attribute can only have one preset value list otherwise it will be changed back and forth and there is no way to validate the correctness of the values.
The relevant list is defined by the connection to the feature which is inside a structure of type "Maintenance" (either primary or secondary maintenance structure), so there can only be one maintenance-connection per attribute.
Typically there is one central maintenance structure where the attributes get maintained and the output structures only get connected to those but do not specify differing values.
In your case, can't you just have 2 attributes "Sale Line Channel A" and "Sale Line Channel B" if they have conflicting value lists?
One other approach would be to investigate into the "different default value", (which is a structure-specific overwrite value for output structures) but this is available in the Desktop UI only.