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