It will not have an impact on the fishes respiration's by using both, they have very different effects on fish. The liquid carbon forms are various dilutions of glutaraldehyde which some fish may not like and is potentially hazardous to you. As long as you use common sense you should be fine. I personally would not use liquid carbon with shrimp or really sensitive fish. There are also several plants that do not like liquid carbon.
Most of the time, I will use liquid carbon to help combat nuisance algae. Pressurized CO2 is much cheaper in the long run, generally.