The StratoLauncher IV 2 x Ultimate STEM Kit will enabling one to launch 2 water rockets employing a single water source (tank sprayer or garden hose) and a single air source (bicycle pump, air compressor, or scuba tank).
The Extender STEM Kit can be added to connect a 3rd, 4th, 5th, etc. StratoLauncher IV Ultimate launcher all using the same water and air sources. Watch the video below starting at the 2:35 minute mark which shows the STEM kit in action.