There ain't no easy way...
I'm in almost the same position. I'm not 100% certain as to how exactly I'll work it, but here's my understanding.
I'm going to call old-fashioned iTunes authorisation "authorising", and the new, extra level of authorisation involved in iTunes in the Cloud "linking". Hopefully that'll help make what I'm talking about clearer!
- Once a device has been linked to either download Previous Purchases, or to Automatically download content purchased on other devices, it can't be set up to do the same for another account for 90 days.
- It goes without saying, each device can only be linked to one account at a time.
- You can log in and out of accounts on any device their authorised for, and make new purchases like you always could. (Though that will reset the current status of the "automatic downloads" toggle, which could get pretty annoying.)
- iTunes cloud to me seems most useful when an account is shared by multiple people who rarely get to physically sync devices to the same computer.
- Remember that if stuff purchased across multiple devices all gets synced to the same computer at home (and all devices are authorised for all accounts), the WiFi syncing feature of iOS 5 will transfer purchased music over your home network without involving the cloud.
So basically, you'll want to carefully choose what device(s) to link to each account, either to set each one up on a different account to maximise access to "distant" content, or to set them all up on the one you use most, to maximise convenience.
In my case for example, one of my accounts has far, far more music than any of the others. It's also used by others to buy. Therefore it's the one I care about getting cloud access to, as this prevents accidental re-purchasing. The others have only me doing the buying, so I don't necessarily need to get access to historic purchases, as I don't delete them unless I don't want them anyway!
Finally, US accounts allow you to get access to historic TV purchases too, which might be something you want to remember when deciding what to link where.
In short, it's a pain! But you can minimise it by choosing accounts carefully.
