Are there, between these two forms, different meanings ?
- They are split up into small pieces.
- They are splitted up into small pieces.
If they are the same, which is more common ?
EDIT - Google ngram for "splitted up": https://goo.gl/hsftzC
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The verb "split" is fully conjugated as shown here. There's no form "splitted." So only your first suggestion is grammatical.
(By the way, it seems to me that you could also omit the word "up," and say "split" instead of "split up." But maybe the context would prove me wrong about that.)