You can find a bunch of manifolds to fit your needs. The easiest way is two needle valves if you photo periods are similar (you can have some CO2 running before the lights go on in one tank, for example, or a bit after). Or you can have a full post regulator for each tank (solenoid, bubble counter, needle valve).
Most people use 1/8 NPT fittings so you don't really even need a manifold. You can just buy a splitter at a hardware store. You really can buy all your fittings at a hardware store and just add your needle valves, solenoids, etc).
You could theoretically just use a splitter if the tanks were the same size and had the same diffusor/reactors. These are cheap and easy to find. There is a chance you could run into problems as the CO2 will take the path of least resistance. I doubt two diffusors would have enough variance to make a big difference, even less with a reactor. But there is always that chance. Add a different amount of plant mass and their could be issues, especially if you are on the cusp of gassing your fish. One mistake and you are in trouble. I would just spend the money on an extra needle valve and manifold/splitter, even on identical tanks.