I'm not sure how things are set up in Floob's image, but you can definitely generate single romlists containing multiple emulators in attract-mode.
All versions of AM can do this from the command line... see the readme file for more info:
In addition to the romlist generation function available in config mode, Attract-Mode can generate a single romlist containing roms for multiple emulators from the command line using the following command:
attract --build-romlist <emulator names...>
With the most recent development version, you can also now generate single romlists containing multiple emulators from the configuration menu: Config->Emulators->Generate Romlist. This will bring up a menu allowing you to set which emulators to include....
Previously the UI only supported generating romlists for a single emulator.
I hope this helps.
so this part:
attract --build-romlist <emulator names...>
do the emulators need to be separated by semicolons? like this:
attract --build-romlist mame2003;nestopia