Can you try with the following code?

query_a = """
SELECT
a.column_a as id_column,
a.column_b as val_column
FROM 
    catalog_a.schema_a.table_a a
"""

query_b = """
SELECT
b.column_a as id_column,
b.column_b as val_column
FROM 
    catalog_b.schema_b.table_b b
"""

@dlt.table
def table_a_data():
    return spark.sql(query_a)

@dlt.table
def table_b_data():
    return spark.sql(query_b)

@dlt.table
def dim_ship():
    return spark.sql("""
    SELECT * FROM table_a_data
    UNION ALL
    SELECT * FROM table_b_data
    """)