After splitting a bill, you can always edit or revert the split check.
Move an order item to another check, by tapping an item and selecting the check you wish to push the item to. You'll notice the screen is very responsive, so moving items between checks is a quick and intuitive process.
You can also split individual order items from the split bill screen. Simply tap on an order item line that you wish to split. A Split item window will open, asking you to specify which checks this item should be split between. When you've selected all the bills this item should be split between, tap Done.
To undo a split item, select the split item to revert and then tap the original check.
If you ever make a mistake, you can always revert the checks to the original state (all order items combined). From the Split bill screen, select the Undo icon on the bottom right of the screen.