I still maintain that "merge to" implies some kind of parent and child relationship.
In other words, I would expect merge topic A to topic B to yield a different result to merging topic B to topic A. The main difference being that the topic being merged "to" is the parent.
But from what you're saying, you expect the result to be the same.
My only issue is that the parent, or very first post, of the topic you're merging "to" isn't upheld if one of the merged posts happens to be newer.
