It really depends on the dog(s). I would assume if you got a puppy, it wouldn't be left loose with your current dog anyway, but in a pen or cage for it's own protection.... so it would probably be the same situation, only now you have 2 dogs to worry about. Also, with your current schedule, a young pup would be extremely difficult. You would definitely need to either get home more or hire some help. An adult dog is much more adjustable and capable of being home alone for longer periods.
Honestly? Our 3 (when I'm staying at my dads house with his 2, and mine) don't do anything when we're not home. They just sleep anyway. And to be honest, they don't really even play that much anymore. I think they all equally would be fine being an only dog. Jackson is when we're at our regular house. Sometimes I think he would like to have a sibling. I know he would, but I also think he's content to have just me as well.
My advice: don't get a dog for your current dog. It hardly works out that way. Get a dog because YOU want one.
Dogs are adaptable creatures. I think as long as Charlie is getting adequade exercise and attention when you are available, all is fine.
