[phyrrus9]

If I'm not mistaken, I believe the default time In MyBB to merge posts from the same user Is 60 minutes. If It's disabled, double-posts will follow.

On the grounds that the feature Is already defaulted, I don't see why It cannot be adjusted.
EDIT: Info added.

Strangely enough if you look at my last thread in suggestions, the posts were 2 minutes apart at most and it didn't automatically merge them.  I'm going to assume it may have to do with me being on my phone, I'm not too sure

If I'm correct with the default settings, the time can be set/adjusted via the ACP. In other words (for example), It may be set at 1 minute, hence all posts within that time-frame generated by the same user will be merged. Anything outside that window, will be double-posted.

This Is something that @Oni needs to clarify.

It's set to a low value. I often do the same as OP, but I usually just type out my post in the reply, copy it and then edit the original post to paste it in.