If you run two canisters into the same diffuser, as the rates decline you will eventually get two bubbles put out at the same time. They will collide, turning into a big bubble, then race to the top smashing and absorbing all bubbles in their path. Most of the gas escapes at the surface. The ladder diffuser is cheap, get two, or get two entire systems.
IME, I was not happy with mixing DIY bottles with the ladder diffuser, somehow the bubbles were too large and much gas escaped at the top unused.
If your tank is near 30 gallons, go on and make or buy a power diffuser and go DIY for the whole thing. Really the first thing to do is to look at your filtration to see if that is causing CO2 to escape. Changing to a canister filter or somehow reducing surface disturbance can make a big difference in CO2 retention.