Most speed controller BECs output regulated 5 volts for receiver and servos; if they are connected in parallel (standard Y connectors) the output is still 5 volts. The only problem that could arise from parallel BECs is when output voltages are not close enough to each other causing regulators to fight each other to try and maintain their output voltages. This the same reason you would not parallel two batteries with different battery voltages or levels of charge.