Quit using the Mac to sync contacts. Use iCloud or GMail to sync your contacts between your devices and computers. You can add both accounts to Mail, Contacts, and Calendars on the Mac as well, and while you can browse them both, they will stay separate on your devices.
Or.
Create separate user accounts on the Mac, and only sync your iPad when signed into the correct account. If you do this your Contacts, Mail, and iTunes libraries and settings will all be different.
In my opinion the first is the better option. It is more versatile and more reliable. The iTunes syncing options are legacy features from before good online services existed.
If you go with the online option, be sure to turn off the contact and calendar syncing in iTunes on the computer first. Failure to do so can bean run away duplicates, as the syncing gets caught in a a loop.
If you're using the same Apple ID, so that you can share apps you can still use different iCloud accounts to keep your data separate. Not an issue with GMail, of course.
