If I understand correctly, the ordering doesn't matter (undated). And 

there is no overlap, so strict complementarity, so to speak.

Then what about:

smpl x --no-missing

series temp = x

smpl y --no-missing --replace

temp = y

smpl --full

series result = temp

(untested, and perhaps some typos included)

--

Alexander Garcia

http://www.usefilm.com/photographer/75943.html