Simultaneous open of two UART on a single NIOS II

Go into the BSP editor, and under the "Advanced" section, increase the value of hal.max_file_descriptors.

Also, it is not strictly necessary to use reduced drivers if you just want non-blocking access. If you're using the full HAL layer, you can also use fcntl() to set O_NONBLOCK on the device. (This does not appear to work on the standard streams - stdin, stdout, stderr - but it should work on a device you have opened with open().) Keep in mind that this will make the device non-blocking for both reads and writes; i.e. check the return value of write() -- don't just assume the character was sent.