In general, Havanese are pretty mild mannered, and it really has more to do with individual dispositions than anything else. That said, The surest bets are either two neutered males or a neutered male followed by a female. MOST two girl households are fine, but I do know a couple of Havanese households where, as the dogs got older, they have needed to be kept apart, or they fight fiercely. In those cases, however, at least one of the pair was an intact female, so hormones probably play a part in the dynamics.
I have an older boy and two youngish girls. The middle girl gets a bit snarky with the other two upon occasion, and it is almost always resource guarding. But even at its worst, it's just a bit of snarling... no actual fighting, and within minutes they are playing again. (And again, one of my girls is intact, and feelings run highest when she is in heat (though she is rarely the instigator).
The MOST important thing is to have your breeder help you choose a laid-back second puppy who is likely to let the older one take the lead. If your first is a boy (which I would advise if you plan to add a second) the girl will USUALLY end up, eventually, being the "boss of him".
But in most cases, he'll be fine with that. Havanese boys, in general, are lovers, not fighters.
. Kodi backs down whenever either of the girls challenges him for a toy or a particular bed. He WON'T back down over food, but even then, he won't fight about it... he is just very, VERY persistent.
No matter what you choose, the VAST majority of Havanese get along very well, especially in households where they are all either neutered or spayed.