I2C setting MCU as both slave and master at the same time
You are not thinking about the problem quite right. You are not setting the MCU to be a slave, you are setting one I2C interface module (one eUSCI) to be a master and another eUSCI to be a slave. The MSP432 itself is neither master nor slave, it just talks to the two eUSCI blocks.

There are two notions of slave address. One is the slave address that the master (you) uses to communicate with some other device, like a temperature sensor. That slave address is used just for the duration of the communication with the particular slave, and then is loaded with a different value when you want to talk to some other sensor or peripheral device. There is no need to store the addresses of all slave devices in eUSCI registers.

The other notion of a slave address is used when your eUSCI is acting as a slave device and some other gadget will be the master. In this case the slave address is more or less permanent and must be defined as part of the initialization, so that the eUSCI can recognize when the external master is sending a message to it.

So, you can treat the two eUSCI modules as being completely independent. Their SCL and SDA signals are independent, and the operation of the two I2C busses can be completely different.