Backing up iBooks to iCloud from two different devices

Hi

This is my current status
---------------------------------------

I have an iPhone 5 and an iPad 2 (iPad used only on wifi)

Both are legit + NO jailbreak etc

both are on ios 7+

Since me and my family use them and we all read different PDF files using iBooks we have different pdf files on each device .

Based on my current usage pattern, It is _not_ possible to have exactly the same files on the iBooks of both devices

I have only one apple id / account which I use to buy my aaps etc. both devices are connected to the same id. apps get synchronised

I back up my phone religiously to iCloud. I haven't backed up the ipad and auto synch is turned _off_ on ipad

there is enough space in iCloud, so this isn't about space


Now my questions
------------------------------
1. since both devices are connected to the same apple id, IF I open auto synch on both devises, (if and when I do that) will backing up one device mean any addition / deletion / change of data on the other device ?

2. alternatively ; is it possible to back up both devices without deleting files i.e. I do not mind all files of iphone's ibooks + ipad's ibooks getting into the iCloud, but I do not want any files / pdf to be deleted

thanks in advance for any help / pointers

best regards


subu