How can I split back the items if by mistake it was merged ?

You can't do that unfortunately. Once an item has been merged, it's been merged, and can't be unmerged.

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Items that spawn other items by tapping at the cost of energy are always better at higher level. They recharge faster and have more storage capacity to hold items when you're offline.

However, items that spawn items automatically, such as the Vase, seem to spawn more items if you have several lower levels instead. For example, I had a level 7,8,9 vase that spawned enough seeds for half all but the last level of the flowers but once merged only gave enough seeds for half that.