Chris,
I expect your rear remotes tap off of the main hydraulic system somewhere. With no other hydraulic activity going on, the rear remotes should be able to flow the full capacity of your pump. I also suspect the remotes rely on the main system pressure relief valve to limit pressure. So, the main system pressure and flow rate should both be available to your PHD.
Disclaimer: My comments are based on my general understanding, not on your tractor specifically. Good luck.
OkieG