You can't do the splitting online. You'd need a stand alone desktop program to do that.
A desktop program would (depending on the specific programs and settings) allow you to run reports on different lines without having to actually maintain separate trees.
You also wouldn't have to re-import to Ancestry.
What you could do is:
1). Do all data entry work in one online tree on Ancestry.
2). Download GEDCOM from Ancestry and import to your desktop program.
3). Run whatever reports you want from Desktop program.
3a). Before importing another GEDCOM download from Ancestry be sure to delete tree from desktop program.
4). When you want new reports just download new GEDCOM from Ancestry and re-import to desktop program.
(Deleting the old import from the desktop program eliminates any chance of duplication and always insures most recent data is used from Ancestry)