FaceTime with 2 Phones (work and private) & 1 Apple ID
Your Apple ID logins on an iOS device are completely independent for these services:


iTunes &amp; App Store
iCloud
iMessage
FaceTime


You could sign into 4 different Apple IDs for these services on one device and that would work fine.

When you say you use the same Apple ID to not have to pay multiple times for iTunes content, that would suggest you would want to use the same Apple ID for iTunes &amp; App Store, then use a different Apple ID between devices for the other three services.

You can find login/logout options for each of these services in Settings. When setting up a new device, you are given the option to use separate Apple IDs for iTunes &amp; App Store by a small link on the iCloud login prompt.