Distinguish between individual Diagrams and the Model as a whole.
Your fundamental job is to capture everything into the Model. The individual diagram are views into the model that let someone understand the model. For a large model it's often helpful to have individual diagrams that just show some related parts. The key is to use a modelling tool that ensures that all the diagrams are consistent - for example if you change the name of an actor in one diagram that change is reflected in the model and hence to all other diagrams.
In your case I would start by putting everything on one page, when that gets a bit crowded split the diagram up. It's quite reasonable to have say, one diagram for one user showing their use cases and then another diagram that pulls together some related use cases used by both actors. The same use case may appear in several diagrams.
Don't focus on the diagrams themselves, focus on capturing and organising the requirements. You want to be sure you have captured all the requirements and can easily find related requriements. Draw the diagrams that help you do that.