I will assume you mean second stock where he did down throw, spike, down throw spike, fthrow regrab fthrow +fsmash
I'll tell you how to easily perform the dthrow to spike, just work on the timing
Grab with Popo, throw down AND press up, then hold forward and up then mash c-stick towards opponent and Popo should throw down, Nana should immediatly as this happens, jump move a little forward, then spike the opponent. Then regrab and repeat.
For the fthrow, dthrow, and bthrow regrabs (not sure if there were any there) its a matter of pressing Z + C-stick in direction at a specific time in the throw, which will cause Nana to dash and grab the opponent. However grabbing from Nana (you have to wait a second or so for her Throw Animation) takes a lot of practice.
I use this all the time to go from a chaingrab to a spike.
Why I wasted my time instead of directing you to a thread. Not sure.