You can't place your second Pilu until the first one is fed porridge and has turned into the green adult Pilu.
Then you can place the baby purple Pilu and start to grow him up.
If you made a second porridge and second green bug, you can then turn this teenage Pilu into an adult green Pilu.
Otherwise, you can leave him as a teenager.
I made two green adult Pilu, and have had nothing but blue stars from them.
Maybe I was too late for the gold nugget glitch that some seemed to enjoy.