I would prefer to merge them if possible as I've been auto logging into both of them and answering and asking questions with both.
Customer Care can arrange for Adobe IDs to be merged, but you need to talk to them directly (phone or chat), so you can prove ownership of both and deal with any transfer of licenses.
User "accounts" on these forums don't really exist, it's just a wrapper for your Adobe ID.
I would simply let sleep one of them. If there are no products attached to them, there is simply no real reason to merge. I personally have 3 accounts for 3 different reasons (private, business1, business2). I could merge the 2 business accounts but I simply think it is not worth the hassle.
I don't think merging account is the idea thing to do and definitely not worth doing why not use one account and forget the other mate!