You could technically 'try' use a single PI controller for multiple inputs by doing a weighted sum of the two values, but this is not recommended. Instead, it is best to have a more complex control system to do this. For conveters, this is often two seperate loops, one being voltage, and one being current. As you are doing frequency control, you could use this approach with some modifications. However, as frequency control is very standardized for grid applications it doesn't make sense to reinvent the wheel. Instead, look at some of the shipping examples that are relevant to your task. They have prebuilt control algorithms in them that you can use as a starting point.