UART buffer, explanation needed

[FrenkR]

hello,
as form documentation, there is a note that only one HW buffer of 128 bytes is used on all three directions(UART0 RX/TY and UART1 TX) of both UART-s. Does this mean that SDK doesn't allow you to simultaneously read from UART0 and write directly to UART/UART1 till read buffer/pointer is emptied(read or flushed)? Or if I am receiving data, I am not allowed to send data on the same UART till buffer is not empty?