According to your post, my understanding is that you wanted to display a single metadata column in the library.
I had tested with this issue, and I had found the following scenario.
If the two content types use the different custom metadata column, it would display two column in the library.
However, if the two content types use the same metadata column, it would display only one column in the library.
Based on your image, did you use the same custom column in the two content types?
If so, did the issue occur in other library? You can create a new library to check whether it works.
You can also check the event log and ULS log to see if anything unexpected occurred.
To check event log, click the Start button and type Event Viewer in the Search box.
For SharePoint 2010, by default, ULS log is at
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\LOGS
If not, how could you create two custom columns with same name?
I recommend you use the same one custom metadata column in the two content types, then check with a new library again.
Thanks & Regards,