is it possible to merge two or more different `collections` and order them by createdAt?

e.g. I have some data like

```
    CollectionA.insert({
        createdAt: new Date(),
        field1: value1,
        field2: value3,
        field3: value2,
    });
    CollectionB.insert({
        createdAt: new Date(),
        field4: value4,
        field5: value5,
        field6: value6,
        field7: value7,
    });

```

I would like to merge and order them on the server that the client get a merged CollectionC later via a subscribe.
How to merge and order the collections? I would like to do this as cheap as possible