Cost of single vs multiple outputs to the same address
A transaction that has multiple output scripts that point to the same address would be no different from a transaction in which every output script pointed to a different address. So when you have two outputs to the same transaction, all that matters is that you have 2 outputs and the size of those outputs (byte-wise).

Transaction priority is calculated as follows:

priority = sum(input_value_in_base_units * input_age)/size_in_bytes

As you can see, only the tx size matters and the input value and age. None of these are influenced by the address to which coins are sent.