Yahoo Pipes: Join feeds in an alternating manner

Question

In Yahoo Pipes its easy to union feeds together (you use the union module) but I want to union two feeds in an alternating manner.

That is, I want to merge this:

Input Feeds:

Feed1:            Feed2:
A0                B0
A1                B1
A2                B2
A3                B3
etc               etc

into this:

Output Feed:

A0
B0
A1
B1
A2
B2
etc

When I use Union I just get A0,A1,A2 ..,B0,B1,B2. I can then sort the result but there isn't anything to sort on that would give me the above.

Is there a way to add an 'index' number element to each field so that I can then sort by that? Or some other way?