To specify a file name via PI marker, you can use a PI marker type called FileName and insert a FileName PI marker in the first paragraph of each part to be split or extracted. Make the content of each FileName PI marker the name you want for the resulting file, without path or extension.
Duplicated file names are hard to locate
Using FileName PI marker can result in two sections of your document having the same name. When this happens the second file overwrites the first, and the first topic does not appear in output. This error is almost impossible to find, unless you search very specifically through all file-name settings and PI markers. Expect many problems of this type if you use FileName PI markers to override DITA2Go-generated file names.