I visited a concert one month ago there and can confirm that the wheelchair places are located behind the standing places and are separated. I can't remember if they are on a higher ground.
I'm from Berlin and Huxleys has just one hall with a capacity of 1.600. The hall with a capacity up to 3.500 is the Columbiahalle. And next to the Columbiahalle is the Columbia Theater with a capacity of 800. Maybe there was a mix-up?
If I select 2 tickets Categorie 1 for saturday, I get the error: "Sorry, there are insufficient tickets to fulfil your selection." 1 ticket would work.