Personally I would recommend you don’t split the sit. Many sitters, me included, would not want to take over from another sitter or have another sitter take over from me.
Have a read of this post:
However, I’m guessing you’ve found sitters to share your sit with. If you haven’t found sitters then you’d need to make that very clear in both sets of dates. Sorry, re your actual question I’m sure one of the moderators will state what you have to do but guess you’d have to end the first set of dates one day and enter the start of the 2nd set the day after the first ends.