Comment 1 for bug 738518

Anyway it should be really easy to merge notifications, see the spec:
- they come from the same program
- they have the same title, or lack of title
- they have the same actions, or lack of actions
- the first is not already open as a fallback alert box
- the first has not already been displayed as a bubble for so long that merging the second into it would cause the newly-calculated duration to be greater than the maximum duration
- both of them have indicated that they allow merging
- the second notification is not specified as a replacement of any other notification.

I think that you just need specify that you allow merging, (and eventually disable the replacement, if you had enabled it).