I'm aware that there's no field in the record format that explicitly relates to co-ownership.
This scenario must come up fairly often--one party owns (is the registrant) the domain name, another party actually operates the Site. In my case: my employer is about to license an e-commerce Site to another outfit. My employer is the current registrant of the domain name.
Obviously, the licensee should have access to the domain name record to re-point the domain name to their own web host, to add/change nameservers, etc., change the admin contact address, change the entire MX record, etc. Still, the licensee is just a licensee not the actual owner/registrant. That's important because for instance if the licensee breaches the license agreement, for instance by selling our competitors' products, my employer has the right to terminate the agreement--but that's not worth much if they can't regain control of the domain name.
A couple of ideas i've had so far:
rely on the 'two-tired' account authority which our Registrar offers (which most others likely do as well), i.e., retain myself as the account's superuser, add the licensee as an admin on the account, so they have full 'working' access to the record;
lock the domain to prevent its transfer by the licensee ('CLIENT TRANSFER PROHIBITED' under the 'status' field), though perhaps with admin access they can change this.
On several occasions, we have purchased domain names from 'professional domain name owners' and used the escrow service provided by our Registrar, and it's worked very well. The intention behind such services is obviously to protect buyer and seller during transaction. A service directed to license-type transactions rather than outright changes of ownership would obviously have to function quite differently, still i thought perhaps such a service might offered by one or more Registrars.
And if you had in mind to reply "that's a legal problem", please don't. It's not a legal problem or a sys admin problem, it's a business problem. And like many business problems it has multiple possible solutions from various functions, legal, IT, and whatnot. The legal solution is embodied in the License Agreement. The Sys Admin solution is directed teh administrative minutiae of the domain name record in order to reconciling the possibly conflicting interests of two parties both of whom (apparently) need access to the same domain name record for different reasons.
Does your DNS registrar not allow you to setup an account for your technical contact? Most registrars do, and this is usually adequate.
You do not have to host your DNS with your registrar. You could delegate authority for the zone to some other DNS hosting service. Then give out access to the hosted DNS control panel without giving out access to the accounts at the registrar.
Interesting question. I think this is more a legal issue than a technical one. In my view the best solution is for your employer to retain ownership of the domain and lease its use to the other party, rather than trying to have co-ownership, as co-ownership is always fraught with problems. If I were placed in the same position I'd be asking lawyers, rather than sysadmins.
I don't see it as a legal issue either. Posession is nine tenths of the law. If I purchase the domain name then I "own" it, regardless of what I do with it. Someone else hosting the domain namespace, web site, or email doesn't infer any "ownership" rights as far as I'm concerned, and my "sub-letting" the domain name to another party doesn't infer any ownership either. To me this is a business\technical issue: What rights to the domain namespace does the business agreement allow for and how will the namespace be managed from a technical perspective? I would say that the customer may need to have the right to add\change\delete A, CNAME, MX, and SPF records but shouldn't have the right to modify the SOA or NS records. At any rate, as the party that registered the domain name, you'll always have ultimate control of it.
If I buy a car and hold the title to it then I can do anything I want with it. I can lend it out or lease it but I retain ownership of the car and retain ultimate control of what happens to it.
I would simply have the new company give you all the settings and have someone from your company make all the needed changes. They don't need access to any of your DNS settings beyond the initial setup. Once everything is setup is shouldn't need to be changed again.