My kittens were both fixed last Tuesday. Bamf is a boy and Pixel is a girl. Bamf was less active than usual for a day or so after the surgery, and he cleaned himself a little more vigorously for a few days after. He was also more affectionate after I brought him home.
Basically, it was a breeze. They have both been great, and it seemed to be much easier on Bamf. I think getting boys neutered is typically very easy on the cat. Occasionally, you will have one like little Omelet, *Pepper*'s baby, who doesn't feel well for a few days after, but most of what I have read implies that they recover within a day or so.
It should be just fine to get them neutered together, and anywhere between 4 and 6 months should be fine. Sooner rather than later, in case they mature too much and start to spray.
They will do great!