as listed in the FSProject Manual:
How to Cascade Multiple FireStudio Interfaces: The first time you add any FireStudio-family interface to your system, please follow these guidelines: 1. Go to www.presonus.com and check for the latest build of the Universal Control application. 2. Connect and sync each of your FireStudio-family interfaces to your computer separately before cascading them. 3. Once a unit is individually connected, launch the Universal Control application. In the Launch window, go to Settings|Check Firmware and verify that each unit has the latest firmware. 4. When the last unit has been updated, connect a second unit to the auxiliary FireWire port on the back of the first unit. Allow the second unit to sync to your computer. 5. Connect additional units to your computer in this manner, one at a time, allowing each to sync before connecting another FireStudio-family interface. 6. Once all your units are connected and synced, launch the Universal Control application. The individual Device Window buttons will be shown in the Launch window in the order of their internal ID number. The unit with the lowest ID number will always be at the top of the list. This unit will also be the first set of inputs and outputs your DAW application sees. The unit with the next highest ID number will be the next set, and so on. Because of this, it is important to cascade your FireStudio interfaces in the proper order from your computer so that you know which input belongs to which unit. Please note: The order of the inputs and outputs in your FireStudio chain cannot be altered. If you are using two or more of the same model (e.g., two FireStudio Projects), you will need to look at the serial number on the bottom of each unit (the internal ID number is the same as the unit’s serial number) and connect them to your computer from lowest to highest. You may also wish to give each unit a custom name. To do so, simply double-click on the name badge on each unit’s Device Window button to open a test box. Once you have typed the custom name, hit the Enter key. If you are using two or more different FireStudio family interfaces, make sure to reconnect them in the order that their individual Device Window buttons appear in the Launch window. Connecting your units in this order will produce the best performance and will put your inputs and outputs in the correct order. It is important to mention that the inputs and outputs on each unit in the chain are independent from one another. This means that you cannot monitor the inputs from the first unit in the chain through the outputs on the second unit in the chain unless you create a mix from within your DAW application. To easily maintain zerolatency mixing between the daisy-chained units, dedicate a set of outputs and a set of inputs from each device in the chain to send a receive a mix in the Universal Control application.
The take away here:
Master units are decided by the SN# and or internal ID and there is not a way to switch which unit is the Master/Slave unit.