According to FIFA rules, can professional players register in more than one club at the same time? or at one season?
Pro players can not be registered with more than one club at the same time. Only amateurs can in some regions.
Transfers between clubs are possible during the season, but limited to registration periods. These are more commonly known as transfer windows and there are usually two per season. The exact times are different for each country and depend on the schedule of the domestic competitions, but usually there is a longer summer period and a short winter period.