You an an individual needle valve for each aquarium. If you try to just split the line, the gas will fallow the path of least resistance.
If the gang valve is built for higher pressure it should work. My experience with gang valves built for aquarium air pumps is that they leak when I use them for CO2 use. Now, you could try yours out. It may just work. Like mentioned above a spit in the line - the CO2 will follow the path of lest resistance. This is true. My line is split in 3 places in line. All 3 has their own brass needle valve. It took a bunch of adjustments to get all 3 adjusted where I want them. It works though!! But after a few months now I am looking at splitters I can attach to the regulator itself so I can have more control of each line.My thought was to put the main line to the feed of a brass gang valve with their own needle valves. Each output from the gang valve would have it's own check valve.