I-Tunes 12 How can I back up two Different I phones

[Crue-leader]

Hi,
I am new here and not very computer savvy. I have an iMac using OS X Yosemite version 10.10.1. My wife has an Iphone 4, which she wants me to back up for her so she can go get the new iPhone 6.

I have an iPhone 5. We both have two different ITunes accounts.

I want to be able to back up both phones independently. I know you used to be bale to switch iTunes accounts by simply holding down the "shift" key while opening iTunes, and there was a window that enabled you to switch between accounts.

That option is no longer available. Is there anyone who can help me with this? I am using iTunes 12.

Thanks
Jim