The use of a splitter (most commonly a T fitting) will not work as pressurized gas will always take the path of least resistance. That means that one tank will receive all the CO2 and the other tank none of the CO2.
You need to have a way to split the output and then have a control (needle valve) for each line.
Ideally you would want a regulator built with this purpose in mind. However since many of us never considered that we bought a mass produced regulator. These regulators can be adapted to work. There are a couple of ways to do so depending on whether or not you want to tear apart the regulator.