I have two accounts. It is a tough call as many value their security. On a computer you can use two different browsers and even go into private mode to prevent sharing of friends. On my Android, I use the Facebook app for him and FriendCaster for Sue and there is no crossover. On my iPad I use the Facebook app and MyPad with no crossover. If anything, I need less devices.
The only time I started to have crossover was when I got a new phone. I saw my niece and daughter listed far down on the suggested friends for Sue. After digging around I discovered I had made a configuration error and was syncing all of my address books from Yahoo and GMail. One fix and the suggestions went away.