the datasheet says... nothing about a second UART, SPI, or TWI interface.
http://www.atmel.com/tools/atmelstudio.aspx?tab=overview...ATmega328PB Part Pack for Atmel Studio 6.2 (37.9 MB, updated July 2015) This bundle adds support for ATmega328PB device in Atmel Studio 6.2.SP2. Please see attached word file for more details.
#define UCSR1A _SFR_MEM8(0xC8) :#define TWBR1 _SFR_MEM8(0xD8)
So I don't understand DrAzzy why you can't find it, it is just a matter of reading it carefully isn't it?!
seems to not be available in DIP layout?