the datasheet says... nothing about a second UART, SPI, or TWI interface.
It mentions them in the "features" section, in table 6-1 "Port Function Multiplexing", and section 15.11 "Power Reduction Register", but not in the individual TWI/USART sections, and the control registers do not appear in the "register summary"... There is no "iom328pb.h" file in the latest Atmel Toolchain that defines anything specific to the PB chip, and it does not appear as a selectable device in the current version of Atmel Studio.
The ATmega168PB, announced at a similar time (Nov, 2014) but more available, does not list extra serial controllers.
Disappointing.
None of the usual distributers have the chips (although several have the Xplained mini eval board.) This is more typical of Atmel; being very slow to ship announced parts. Usually their documentation is in better shape during the "I want that and I can't get it yet" phase. 