This script will combine two or more selected tasks. The first one is kept as the base and the rest are appended and then deleted. I haven't figured out how to retain any formatting that may be in the notes so the new task will be all plain text if that matters to you.
Code:
-- Combine two or more selected tasks into one
tell application "OmniFocus"
tell document window 1 of default document
set _trees to selected trees of content
if (count of _trees) < 2 then
display dialog "You must have more than one task selected"
else
set _tasksToDelete to {}
set _masterTask to value of item 1 of _trees
set _names to "From: " & name of _masterTask
set _notes to name of _masterTask & return & note of _masterTask
repeat with i from 2 to count of _trees
set _task to value of item i of _trees
set end of _tasksToDelete to _task
set _names to _names & return & "From: " & name of _task
if note of _task is not "" then
set _notes to _notes & return & return & name of _task & return & note of _task
end if
end repeat
set note of _masterTask to _names & return & return & _notes
repeat with _task in _tasksToDelete
delete _task
end repeat
end if
end tell
end tell