IEnumerable<TabTransaktion> allTransactions
    = TabTransaktions1.Union(TabTransaktions2)
                      .Where(trans => trans.TabVorgang != null).
                       .OrderBy(tran => tran.TabVorgang.Wirkdatum)
                      .OrderByDescending(trans2 => trans2.TabVorgang.ID);

SELECT BusinessEntityID, SalariedFlag
FROM HumanResources.Employee
ORDER BY CASE SalariedFlag WHEN 1 THEN BusinessEntityID END DESC
        ,CASE WHEN SalariedFlag = 0 THEN BusinessEntityID END;
GO

IEnumerable<TabTransaktion> mainQuery
    = TabTransaktions1.Union(TabTransaktions2)
                      .Where(trans => trans.TabVorgang != null);

var queryOne  =  mainQuery.Where(p=>p.SalariedFlag ==1)
                       .OrderByDescending(tran => tran.BusinessEntityID );

var queryTwo  =  mainQuery.Where(p=>p.SalariedFlag ==0)
                       .OrderBy(tran => tran.BusinessEntityID);

var finalResult = queryOne.Union(queryTwo);