I2C-communication is blocking. I'm not familiar with the low-level-details of the I2C-Bus.
My guessing is If the number of bytes does not fit to the request this makes the I2C-bus hang.
Though it seems you are using the right numbers.
nodeMCU means an ESP8266-nodeMCU?
If your master is not using anything else you could simply define two softwareserial interfaces.
What is the maximum datalength in bytes that you want to transfer in your final application?
What is the required frequency that you send data from the slaves to the master?
What sensors are connected to the slaves? I mean what interface do the sensors have to communicate with the slave?
best regards Stefan