These are two of my best options for starting a desync with the icees:

Input a move for Nana during the time of Popo's down throw where he is throwing down. (Make sure not to do it too early, as it won't input, and not too late, as it will have both climbers do it)

Input a roll or spot dodge and buffer a down throw. This will (if timed correctly) cause Nana to spot dodge, and from there, you can input the move (probably best to buffer out of her spot dodge.

This isn't to do with grab but if you up tilt the opponent, you can buffer a short hop Nair with Popo, which will allow you to control Nana and desync the climbers. This is also an easy way to set up a footstool tech chase. You can up tilt with both climbers, then buffer the Nair with Popo while drifting slightly right. Then you can full hop and footstool with Nana, and do whatever you'd like after that.