Sharing apps is actually pretty easy.
On your iDevice, just go to Settings > Store and sign into the account you want to download purchased apps from, and then when you go to install them, it will act like it's going to purchase them again, start to download, and then pop up a dialog saying you've already purchased this app, click OK to download again for Free.
I do this on a weekly basis with 2 other accounts and have been doing it for over a year so I know it works. Even on iOS 5.0.
Just be careful to switch to the right accounts when you make your purchases. Since it does not necessarily say that you have already purchased the app, you could look up the wrong app (a HD universal version instead of the normal version, for instance) and end up purchasing it by mistake.
Finally, note that you could sign into your alternate Store account, then go to App Store > Updates > Purchased and see a list of what's available for download on that account, BUT NOTICE that this will LOCK your device to THAT account's past Purchases for the next 90 days!
See: http://support.apple.com/kb/HT4627
"When you turn on Automatic Downloads or download past purchases on an iOS device or computer, your device or computer will be associated with your Apple ID. Your Apple ID can have up to 10 devices and computers (combined) associated with it. Each computer must also be authorized using the same Apple ID. Once a device or computer is associated with your Apple ID, you cannot associate that device or computer with another Apple ID for 90 days."
Here's our setup:
Computers:
- iMac x 1 (wife's primary computer & used by the kids)
- MacBook Air x 1 (my primary computer)
- MacBook Pro x 1 (wife's)
iOS Devices:
- iPhone 4 x 2 (wife's and mine)
- iPhone 3G x 1 (Younger Daughter's, used in airplane mode)
- iPod Touch x 1 (Older Daughter's)
- iPad 2 x 1 (mine but can be used by all when it's at home)
- Apple TV 1st Generation x 1 (used in kid's playroom)
- Apple TV 2nd Generation x 1 (used in main living room)
The Setup:
We use one Apple ID for all iTunes purchases (music, videos, books & apps) across all devices. Each user of an iOS device also has their own unique Apple ID used for iCloud.
The iMac is our main hub. All music and videos are stored on it. All iOS devices with the exception of any that are mine sync with the iMac. I maintain my own music on the MacBook Air that I manually manage (I will copy over items from the iMac that I want on to the Air and will copy over times that I have added to the Air back to the iMac - I believe that this can be automated with the use of Home Sharing when both computers are on the same network). All iOS devices that are mine sync with the my Air.
Music on the kids's and wife's iOS device are managed individually via syncing with the iMac. This goes for videos as well, however we usually only access videos when we are at home and therefor rely on the built-in Home Sharing ability of each iOS device to stream content from the iMac to the device. This is also how we use the 2nd gen Apple TV.
We have no iTunes setup on the MacBook Pro as my wife uses the Apple TV or her iPhone to listen to music and only uses the laptop for web browsing.
The 1st gen Apple TV has been jail broken and has had its hard drive upgraded. I manually copy over videos to it for the kids (only non-iTunes purchased videos).
More info on Home Sharing: http://www.apple.com/support/homesharing/getstarted/
On Merging Apple IDs:
This cannot be done: http://support.apple.com/kb/HE37 (5th question down).
Best Answer
Short answer: Dropbox (preferably with Dropbox-aware apps).
Longer answer: It depends a lot on what type the documents are, and what apps you use to edit them.
Anything that only has iCloud as a sharing mechanism won't work, without exporting via email or some other means (which would seem to defeat the point). For the foreseeable future, iCloud documents are tied to individual accounts.
Dropbox is probably your best bet - either a shared account, or shared folders between individual accounts. The app itself can open files in whatever application supports them, but then you're dealing with multiple versions again. If at all possible, use apps that have built-in Dropbox support to avoid this.
If you're dealing with plain text, there is a vast selection of Dropbox-enabled text editors out there. Brett Terpstra has a heavily detailed comparison here.
For other formats, it may be a bit more hit and miss. For Office documents, Office2 HD and Documents to Go both advertise Dropbox sync, but there may be more out there.
For PDFs, PDFPen does Dropbox sharing, and is a great app. PDF Expert has similar functionality.
If you narrow down what sort of document support you need, I'm sure others can chime in with suggestions.