you could use a microcontroller with multiple hardware serial ports as the hub, e.g. Arduino Mega
the application and nanos transmit packets to the Mega indicating the destination of the data
the Mega forwards the packets to the appropriate destination
how fast do you require the communication ?
if the nanos are transmitting to the hub do you also require serial monitor communications
any particular reason to be using two applications and two nanos?
won't one more powerful microcontroller do the work?
edit: a diagram showing the data flows may help?