To manually fire multiple modules at the same time, place as many modules as desired on the same channel. When you fire a cue on this channel, all modules will fire simultaneously. Please note that when there is multiple modules on the same channel remote continuity checks function differently. To find out more please see How do I test continuity on multiple modules on the same channel?
For scripted firing (including STEP scripting), you can also fire multiple channels at the same time. Firmware version 2.1.1 or earlier has a limit of 2 channels firing in any 0.4-second window whereas 3.0.0 or higher has no limit to the number of channels you can fire at the same time.
The unlimited channel scripting capability in 3.0.0 or higher is, however, limited to the first script in the file. Any script beyond the first is limited to the same restrictions in firmware versions 2.1.1 or earlier.