We often do adopt two kittens at once and we have for 15 years with great success.
However we would always recommend one at a time .
This is because on going to a new home, a kitten with a brother or sister will have less need to bond to the new family.
They already have there comfort blanket with there litter mate and often they do not become humanised as they would going as singleton .
We recommend introducing a second kitten 3 months later of the other sex .
However we are happy to adopt two at a time as long as you aware of this issue and have time to socialise both very well, this isn’t always but it is something to consider.