It would be convenient to be able to zip two bags of equal length and partition-lengths together.

This would be particularly handy when splitting and joining bags from an original bag

b = db.from_filenames(...)...
x = b.pluck('x').map(...)
y = b.pluck('y').map(...)
z = b.pluck('z').map(...)
out = db.zip(x, y, z)

We might want to be careful to check that the lengths of the interior partitions are the same.

https://groups.google.com/a/continuum.io/d/topic/blaze-dev/NkeyFrOtcWo/discussion