How does that work exactly? Do you need to switch to a different active iCloud user?
No. I think the confusion in all this stems from the legacy use of AppleIDs and the current understanding of iCloud
iCloud is really two services right.
1) General iCloud - can be any AppleID you make it (doesn't have to be @m3 account). This will then serve as your account that will backup your settings, device, do FindMyIphone and Photostream and contacts/Cal.
2) Perosonal iCloud - this is email (requires @me)
When you set up iPhone for first time, if you don't have an iCloud account i would set up a new one at that time. If you already have a MM account, i would enter that info into the iCloud setup and it will convert this account to an iCloud account.
Next I would then go into Settings and under the Store setting, enter whatever AppleID is used for purchases. Could be same as your MM or a different one but its obviously not the new iCloud one you set up.
Now that you have done that for person A, do the same thing for person B only their iCloud account will be different.
Each device now has a main iCloud account that serves as the master account for iCloud backups etc. However, when it comes to email, contacts, calendar etc, these can all be shared across devices. So assuming person A signs up for iCloud or converts MM, they now have the ability to manage email, contacts, and calendar on their device. This will be tied to the iCloud account (or should be in other words if you want an @me, make it your iCloud). Now if person B does the same thing, you can add person B's iCloud info (although really its could just their @me info and they are kind of synonymous) to person A's phone. You will see the ability to turn on person B's calendar, contacts, etc. However, person A's iCloud account still manages the device.
Does that make sense? Its confusing because of the separation of Personal iCloud (which is the email and people need an @me) from General iCloud. But its all being called iCloud.