If you also wanted to preserve formatting you could back-up your data and then experiment with something like the following:
Code:
tell application id "com.omnigroup.omnifocus"
tell content of front document window of front document
set lstTasks to value of selected trees where (class of its value is task)
if length of lstTasks < 2 then return
set oFirstTask to first item of lstTasks
set lstRest to items 2 thru end of lstTasks
tell oFirstTask -- INSERT TITLE OF FIRST TASK AT START OF ITS OWN NOTE
insert name & return & return at before characters of note
duplicate last character of note to after last character of note
insert return & return at before last character of note
delete last character of note
repeat with oTask in lstRest -- APPEND TITLES AND NOTES OF OTHER TASKS TO NOTE OF FIRST TASK
duplicate last character of note to after last character of note
insert ((return & return & (name of oTask) as string) & return & return) at before (last character of note)
delete last character of note
duplicate (characters of note of oTask) to after (characters of note)
delete oTask
end repeat
end tell
end tell
end tell