we use 2 on our static, (4040) you should not put 1 after the other! as this does not speed up production and just feeds water with a very high tds into the second membrane.
buy 3 y pieces and split the inlet, pure, and waste pipes, and as sugested use a booster pump. 100psi. now you will see more water produced.
another option if you have a 4040 already would be to contact gaps water and replace your membrane with their low pressure HF5 membrane, bar far the best on the market, these are designed to produce water fast at low pressures like 30 psi, so if your water pressure is 50 or above these are fantastic.