同時做兩間公司董事

As a matter of ethics and, in particular, the same business carried out by A and B, you have to avoid the issue of conflict of interest. You have to let all parties know that you are acting as a director of their company. There is no restriction in Companies Ordinance that a person cannot act as a director for over 1 company.
So, declare your postion to all parties and if they know it and not put on any question on you and, you are acting in good faith, it will be OK for you to do so.