The problem you are running up against here is the DreamHost Promo Code programs rules that are designed to keep users from perpetually "churning" their domains through "new accounts with Promo Codes and avoiding ever paying DreamHost's (very reasonable, IMO) standard hosting rates.
The matter is even further complicated in that your "free" domains are only "free" while you host with DreamHost, hence their "freeness" is forever tied to their hosted relationship under that "initial account". Live and learn, I suppose.
None of this has anything to do, really, with the domain registrant ("owner"); you can "transfer" that at any time to another registrar (once you whois information is properly set), at which time you will have effectively separated them from the initial DH account. That account will now have "credit" allowing the registration of different "free" domains (to replace the ones transferred - note that is registration transfers) and those registrations will be "free" while the hosting plan is continued or until those domains registrations are transferred.
Your problem is the "joint venture" nature of your relationship with your partner on the management of that account. My suggestion is that you resolve that issue and accrue to yourself sole management of the account - that is the only way I see for you to prevent "anyone else to have the ability to change/delete anything dealing with those other domains".
While I understand your situation a little better now, I still think you are possibly making it harder than it needs to be, and might be related to trying to "max" the "freebies" associated with the Promo at the expense of adequate control/flexibility of your domain registration/hosting. part of the confusion stems from your use of "transfer" and "keep". As I said before, you can always modify the whois records so that only you can manage the domains in question, and then "transfer" their registration as you wish.
Note that this "transfer" is only involving domain ownership/management, and has nothing to do with any "transfer" of "hosting" between accounts/plans on DreamHost, or to any other potential host - that is another matter altogether .
ICANN rules are your protection there, as they require the participation/approval of the registrant or the administrative contact to effect transfers of ownership, registrars, or whois info. If you are the registrant, the billing contact, and the Administrative contact, you are "protected" in that only you can authorize or order registration changes.
Well, not really. Being "hosted" is more a description of an arrangement between you and a hosting organization than it is any such DB/upload/files presence distinction. When you "add" a domain to be "hosted" panel (or when it was added "automagically" in the setup of a hosting plan) and DH establishes a DNS record for it, it is "hosted" for the purposes of the DH rule regarding "inter-account" transfers.
The substance of the issue is that DH has to make sure that:
1) Only the correct "account holder", or their designee (see "priviliges") manipulates the features of the account
2) Registrations are handled according to ICANN rules
3) The Promo Code benefit is used as intended: to give a "one time cheap preview" to someone wanting to check out DreamHost hosting.
They figure that once you have hosted a comain on DH, whether under another's account or one you previously owned, you know the value afforded and can then decide whether their fees are justified. It's the "cheap" bastards that will always try to "game" anything they can for "free" that caused them to implement this "once it is hosted here it can never be 'new" again" policy regarding domain hosting plans - but that has nothing to do with ICANN rules for transferring domain ownership.
Anecdotal evidence on these forums, and in the IRC channel, indicates that if you discuss this situation candidly with DH via the support system, and agree that "unearned" or "inappropriate" promo code usage is reversed or forgone, they may make an exception to the rule and allow an inter-account transfer of a previously hosted domain. This presume that only one of those accounts was created using a promo code, and people report having "mixed" success with such requests. I'm sure they will work with you as best they can.
The real arbiter of all of this is the DH staff; I'm just sharing what I have learned via interaction with others on these forums - you should initiate a dialog with DH on this whole issue, so that you can get your issues authoritatively addressed.