If you have the mosquitto broker running in another pi, you don’t need to install mosquitto on the openhabian pi too.
Install the mqtt binding in the paperUI
This will create a file called mqtt.cfg located in the config/services folder
Edit this file to include the details of your already running broker and OH will connect to that broker
See:
https://www.openhab.org/addons/bindings/mqtt1/
and:
By the way, I use this set-up too, I have a mosquitto broker running alone on a pi-zero. That’s all it is running. And OH is running on a recycled HP thin client.