That is really odd to see kittens having so much trouble getting along. It may mean that one or the other was not socialized to other cats, which is a problem. One other explanation may be that they are a little bit traumatized by the neutering, in addition to being re-homed.
I have to believe that this will work out well, but you probably have to "start over" and go slow. I would keep them completely apart for a few days. Let them get their bearings and start to feel better from the operation etc. The goals are to: a) make sure they are 100% comfortable in their new diggs, b) get them used to the idea that there is another kitten sharing their home, c) develop a positive association with the other cat, d) bond with you.
So keep them apart for a few days, and at least give the shy one a "safe room" to stay in. Give them each their own litterboxes, food dishes, water dishes and toys. Play with each of them separately. Then I would try some short supervised visits and see how it goes. The moment there is hissing or growling I would abort the visit and wait until later that day or the next day. If it goes poorly, you may have to start over, and do the full cat introduction protocol, which is basically to get them each used to each other's scent without the other cat being present (site swapping, or scent swapping - have them change places as far as where they stay separately, or take cloth items containing the other's scent and put that in the room with the other cat). To build up positive associations you can feed them such that they are close by but can't interract. So on opposite sides or a door, opposite sides of a hallway, or some other barrier in between them. Then when the meal is over, separate them again. This makes them associate something extremely positive (meal time) with the other cat. You repeat this each meal until the growling or hissing is completely gone.
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