Once you upload the 1st document it should take you back to the "Add your docs" page. You can then scroll down and click on "Add Another Doc" on the bottom right, and then click "Share Now" to add another document.
As for the income/expense sheet - is it a large file? If so see if there's a way you can lower the file size. Or maybe try saving it as another file type like a .csv or even a .pdf.
If you are still having problems with uploading your files, just do add as much as you can and then keep going. You can talk to a tax prep assistant for help later.