var q1 = s.Query.All<MyEntity>().Select(a => new { a.Text, a.Key });
var q2 = s.Query.All<AnotherEntity>().Select(a => new { a.Text, a.Key });

var union = q1.Union(q2);

foreach (var item in union)
{
    Console.WriteLine("{0}, {1}", item.Text, item.Key.Format());
}

in this example all keys is formated as MyEntity keys