Syncing two separate iPhones

Question

I can't find any Apple-approved guidance or information on what happens when one tries to sync two iPhones to the same computer. If there is and I've missed, please redirect me and disregard the following.
1. If iPhoneX is activated under one account and iPhoneY is activated under another, will there be any problems with iPhoneY syncing music that was purchased under the account associated with iPhoneX?
2. iPhoneX doesn't currently sync anything (except for media) with iTunes: http://www.flickr.com/photos/50972001@N00/2123609143/ As far as I know, the Contacts, Calendars, Mail Accounts, and Web Browser for iPhoneX are either stored in iTunes itself, or not backed up at all and are solely on the iPhone.
If this information is actually stored on the computer somewhere, will iTunes differentiate between iPhoneX's content and iPhoneY's content?
3. This is dependent on the last question's answer: If I would like to selectively have some (but not all) of the contacts from iPhoneX copied to iPhoneY, for the sake of efficiency can this be done?
Any and all help would be appreciated! iPhoneY is already purchased, and I would like to activate and sync it appropriately for Christmas, but need to understand exactly /how/ that process will work before I do so. Thank you!

Answer 1

Answer to number 1:
You can Sync iPhone X and Y to the same computer without any issues. As long as your both into the same music, videos, contacts, etc.
If your tastes are really different I advise just creating another user account.
As far as purchased content. Any user can download this onto their iPhone but it is limited to 5 total iPhones, iPods, and or Computers. So you would need to Authorize both iPhones to have permission to play your purchased content. So you would then be using 2 out of your allowed 5.
About iTunes Store authorization and de-authorization
http://docs.info.apple.com/article.html?artnum=93014
The iPhones can be activated on any PC/Mac and do not have to return back to that PC/Mac ever again even if you get a new iPhone. We allow activation on any computer with iTunes 7.5 and a valid Internet connection.
Answer to number 2:
Each iPhone has it's own prefernce. For example iPhone X only wants to do music because it is a music lover and could care less about anything else. If iPhone Y came into the picture. It as well could Sync music or anything else in the world. Whatever you do in the Summary, Info, Music etc. Tab for each iPhone sticks to that iPhone and has no affect on the other iPhone.
Think of it like you both having your own car and presetting the stereo to what you want. The other does not have to worry about going into that car that does not belong to them and listening to a radio station that they may dislike.
Answer to number 3:
The only way to can narrow down what contacts go to each iPhone. Is assuming your Contact program allowed Groups. If it does you could create iPhone X contact group, and iPhone Y contact group. The equivalent to you both having your own play list.
Again this feature is dependent on weather your Contact program provides this feature.
If you would like. Call 1-800-My-iPhone or your local iPhone support and an Agent can assist and answer any other questions you might have.
The best all out option would be you both having your own user account so it's like you both having your own computers.