A 'normal' Union returns a result that contains all rows from both sets with duplicate rows removed. In order to remove the duplicate rows Data Migrator must SORT both input sources, and it is here where you encounter the DM has a maximum of 128 sort fields.
The easiest is if you are sure that the 2 sources do not have duplicate rows, or you do not mind to have those duplicates. In that case change the Union properties to Union Type = Union All
If you do need to remove the duplicate rows there are 2 solutions:
If the Target is loaded as Insert/Update with Update if the record exists, this update will remove the duplicates.
Otherwise I suggest not to use the DataMigrator Union, but make a DBMS SQL flow by which you let the DBMS do the UNION (and remove the duplicates) in stead of DataMigrator.
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