you don't need to get some CAN stuff for that. TWO WIRES only

One scheme... there's a thousands other way but.. I2C could be interesting also

1 Busy line
1 TX/RX line


Master enable the busy line
all slave listen the Master for a x timeout delay
Master message done, Master disable busy line and wait reply of a Slave
Slave enable busy line and talk to Master


That imply to switch I/Os to input and output but this work...

OR if you want to do everything more easy.... another scheme


1. TX (From master TX to all slave RX)
1. RX (From all slave TX to Master RX with diode mixer and pull down/pull up resistor depending of your serial mode (inverted or true driven))