set inFile1 to "yourVideo1.avi"
set inFile2 to "yourVideo2.avi"
tell application "QuickTime Player"
make new document
open inFile1
tell document 1
rewind
select all
copy
select none
end tell
close document 1 saving no
tell document 1
add
select none
end tell
open inFile2
tell document 1
rewind
select all
copy
select none
end tell
close document 1 saving no
tell document 1
add
select none
end tell
export document 1 to "yourNewVideo.avi" as AVI
end tell