Diiiiid someone say FRAMES?!?!?!? Let's go! :D
Fair and Bair are easiest here. SH Fair and Bair give you a two frame window if you don't FF. If you FF the aerial it can lessen to a one frame window (AKA, frame perfect). If you use other aerials (such as Nair and Dair) it is usually a 1 frame window,
and can become impossible depending on when you FF. Use SH non FF Fair or risk inconsistency IMO =[
You can actually start your aerial iceblock before Nana is ready, so you need to use a minor delay (unless you want a solo popo aerial iceblock, which is very beneficial in its own right).
This window is dependent upon a few different things.
Let's say you jump frame perfectly as Popo and initiate the double iceblock as early as possible.
7 Frames. You have 7 frames to do something as Popo after landing before Nana is influenced. This is a really nice window, but you have to know the special landing and iceblock animation decently well to tell where it is.
However, if anyone feels this is too hard, you can delay your double iceblock. Popo will go into special landing regardless, but the larger the delay before the iceblock, the longer Nana will be in animation after you land, making that window up to 3 frames larger it seems.
However, if your jump out of the L-cancel desync is late, you will decrease your window. This window can be decreased by 7 frames.
Also if you make Popo do his aerial iceblock quickly, he won't get a rise out of the iceblock, letting popo land more quickly and increasing the window between him and Nana. This increases your window by 10 frames.
The largest window that can be attained is 42 frames (aerial iceblock with Popo before Nana is out of lag, then iceblock with Nana super late). I would say it's unideal for Popo to immediate iceblock, and delaying Nana's iceblock seems bad as well, but there's a lot of small options that add up to this 42 frame window.
I don't think anyone has anything to worry about here. The main thins is jumping ASAP after the L-cancel desync, then just wait and iceblock. You will have a relatively large desync window after landing to attack with Popo first.
EDIT: I made a thing
http://gfycat.com/BelatedSelfassuredArmednylonshrimp
EDIT: and another thing to show the desync after landing
http://gfycat.com/VagueDefensiveAvocet