Is it possible to run 2 functions, that communicate with 2 different serial devices, in parallel using FreeRtos library?
It is unlikely that FreeRTOS will allow you to do that. SoftwareSerial uses busy loops with interrupts disabled, which means it cannot be interrupted or preempted by the RTOS (or by anything for that matter).

If you want two serial connections in parallel, at least one of them should be done in hardware.