There are ways you can practice petplay alone. I think, however, that it depends on what you're trying to get out of it - if you want the emotional experience of being controlled by another person, having a partner is going to be more-or-less essential. But if you don't mind just acting like a pup - I know not everyone plays as a canine but I do - it's pretty easy to do alone. I've had sessions where I wear ears, tail, and collar and tie myself up with a leash, while having a bowl of water to lap from, and even a dog toy to chew on or a bowl of dog food to eat. And in my experience it tends to feel... rather natural, really. Not weird or anything. I suppose kind of the same feeling I get from doing baby things.