Apparently, if you trade away a Zygarde of any %, you will lose however many cells it took to construct it. See here:
Creating a zygarde will reduce the current percentage in your zygarde box. Trading this will not refund any cells, and you are unable to re-acquire them.
I wonder traded 2 10% forms, and have lost that 20%.
On the flip side, you are able to have more than 100 Zygarde cells. From this discussion about what happens if you break down a Zygarde you received in a trade:
Okay, I tested it out. It doesn't register you as having more than X/100 Zygarde, but the game acts like you do anyway. My Zygarde now has the Power Construct ability.