Multiple Certificates/Provisioning Profiles in one Xcode organizer?

Question

There are several similar questions here, but none could answer my basic question:

Is it possible to have two separate developer certificates in the keychain and two corresponding distribution profiles in Organizer?

I have my own (working) developer certificate and provisioning profile for my own iPhone apps. Now I finished a project for a client and would like to use his developer account/certificates/provisioning profiles to submit his app to the app store on his account.

Is that even possible?

So far I downloaded and installed his certificates and they show up in my keychain, and I installed his provisioning profile in Organizer, but Organizer tells me "A valid signing identity matching this profile could not be found in your keychain.". Likewise, the archive build fails.

I couldn't find a way to tell Organizer to use the certificate of my client instead of my own-I think this is the problem.

Thanks for any help!