Hi Steffen,
Yes, two USART are available on the extension connector :
USART2 on PA2/PA3, signals named Cx_UASRT_Tx/Cx_USART_Rx,
USART1 on PB6/PB7, signals named Cx_I2SCL/Cx_IS2SDA.
For use the second one, you must configure the port by remaping.
(Take a look at the "STM32F103xC/D/E datasheet" and STM32 reference manual on ST website).
The Irda is also connected to USART1 but on PA9/PA10, and it is not managed by CircleOS, it is only available for applications.
So you can use the two USART with your own application.
Yves