ToggleBetweenTwoOpenDocuments

set document1File to POSIX file "/Users/jesse/Documents/document1.taskpaper"
set document2File to POSIX file "/Users/jesse/Documents/document2.taskpaper"

tell application "TaskPaper"
    set currentDocumentFile to file of front document

    if currentDocumentFile = document1File then
        set nextDocument to document2File
    else
        set nextDocument to document1File
    end if

    save front document
    close front document
    open nextDocument
end tell