Is it permissible to deal with a brokerage company that charges commission from both parties (the citizen and the companies) for buying shares in non-usurious companies such as real estate and telecommunications, knowing that it also deals with usurious companies?

There is no harm in dealing with the company in permissible matters, though it is better to avoid dealing with it if there is another company whose work is limited to mediating the sale of permissible shares, because whoever openly commits a forbidden act, such as mediating the sale of interest-bearing shares or other illicit things, must be condemned, and the least of it is to boycott him until he desists and repents.