I use one foot at a time for calf raises off the leg press because it is easier for me to achieve overload than if I use two feet at a time. I would not say one foot at a time is any more effective, but for me, it is definitely more efficient.
You need to overload your calves just as you do any other muscle to achieve maximum development. For me to overload the calves in a 6-8 rep range, it requires me to load the leg press up to the max and then some. I’ve found I can make better use of my time and energy by doing one foot at a time and cutting my loading and unloading of weights in half. I’m putting all my energy and intensity into my training and not loading the machine.
If you can effectively and efficiently overload your calves using both feet at the same time off the leg press, there are no problems with continuing to execute the exercise that way. In fact, that’s more true to the Max-OT principles because you are training both calves at the same time. You should always strive to achieve the maximum amount of overload as efficiently as possible.