The key to the problem you are having is that you created the db/host (subdomain) in question before the configuration change that allows you to install multiple DBs in the same subdomain or "host" instance.
I have the same situation. The only sub-domains where you can do this are ones that were created after the change. Josh talks about this on the Dreamhost Blog back in March, and further clarifies it in the comments/discussion (see the above link).
That depends on whether you move the DB from the "old" domain you created before the "many DB/one host" change to a subdomain you created after the "change". There is more clarification and discussion of the pros and cons of all that on the blog at the link I provided.
The only point that I can see is to have both DB's available from the same instance of phpMyAdmin and have the multiple databases on the same server. Having them on the same server can be "good" or "bad", depending on your point of view. Again, this is discussed further, and commented on, in the blog entry I linked and the comments. I don't mind having my databases scattered all over, and have not felt it was worth the trouble to "re-do" stuff just to get them all on the same box, or see them all from the same phpMyAdmin url. Of course, YMMV.
If you really want to "put them all on the same domain". just Create a "new" sub-domain, export/backup the "old" database, then delete the old database. It will now be "out" of the system. Then create it "over" again, add it to the "new" sub-domain you created, and import the data that you saved from the old database. Now the databases will be on the same sub-domain. Then go change all your scripts to point to the new host as appropriate.