Bumping up this suggestion to gain attention/possible inclusion in XF2, it's a simple feature that needs to be in the core to have double posts merged and not stay separate or at least controllable by admins. Add-ons exist for it currently, but other products have similar feature built in for ease of use and moderation.
While the first post in this suggestion is vague and doesn't explain what function is wanted, I am still surprised that so few people voted for this.
The addon providing this function is one of the top downloaded addons.
It is one of the pet hates of admins to see a spree of back to back posts by the same member.
Please implement a function that merges subsequent posts by the same member if they are made within a certain time span (X hours).