I was toying around in training mode and think I've found a decent desynch.
As you are landing, hold L/R and 'Up' on the control stick. They both shield, but Nana jumps out of it, so you are left with Popo shielding and Nana in the air.
It functions exactly the same way as holding X/Y after you land to have Nana continually jump, but the difference is that Popo will immediately start to shield.
Nana jumps out of the shield on the first possible frame, and she can do any move by herself (by using the joystick or c-stick, Popo will roll/spotdodge, so neutral or up moves work best). If you Ice Block right after Nana leaves the ground, her jump is cancelled and she'll stay closer to the ground than a normal SH. This desynch, although situational, seems to have good speed and is easy to use.
Also an unrelated tidbit, it appears that Nana becomes unresponsive when she's just about to land (in air, slightly above ground). Therefore, you can have Popo do any move right before you land to desynch. However, most moves' starting lag is too long for the hitbox to come out in that small window of time. Ice Block props Popo up which allows him to fully Ice Block before landing (aka Landing Ice Block desynch), but I've found d-air can actually work as well. You'd probably never actually make contact with the d-air, but it reduces the amount of time Popo has to spend in the air.
Could someone with AR check up on that (if possible)? If that reasoning is correct, how many frames before landing are open, etc.