I think it depends on you, your family situation, and your puppy's temperament, as well as the puppy you are considering getting. My baby Ceylon (15 weeks) is doing great on his own with my daughter and me, and although I am thinking that I probably will want to get him a companion at some point later when he gets a bit older (like, in a couple of years or so, maybe a rescue hav), he does just great on his own, and for now, I really want to foster his ability to be OK alone when he has to be alone, and also I want him to grow up depending on us humans first for love and companionship.
I am not saying that getting a companion for your puppy is the wrong thing to do - only you know what your puppy needs, and whether she would benefit, or not, from having another puppy to play with all day long while you are gone - but in my mind, 12 weeks is too young for deciding anything, really - at 12 weeks, if they have been socialized properly, they will be just fine with almost any situation presented to them. How long have you had her? Puppies take longer than a month or two to really 'settle in' - - and if you do take the other puppy, you do
know that you are assuming just as great a responsibility with the new puppy, as you did when taking your Havanna in the first place...
Only you can decide if the added responsibility is worthwhile. Myself, with a 12-week-old puppy, I would give it more time - but I don't know your Havanna, or your situation, so I can't judge you either way
. I do want to know what you decide though!