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Questions &/or issues related to products in the LCDs and OLEDs category should be posted in this forum.
The display IS an Arduino, but any microprocessor/computer can send it information to display.

You could use the display as an Arduino for standalone projects (reprogramming it with the Arduino IDE), but it gives easy access to only a very few pins for external I/O connections.
I would need it to talk to two buttons, a second display and a force resistor. Does it have enough pins for that or would I need a separate arduino? I apologize - Im new to electronics and its not obvious to me from the schematic.
Yes, there are enough I/O pins on the display to do that. You will have to remove/rewrite most of the firmware in one of the displays, in order to make it capable of handling the buttons, the FSR and send info to another display.

There is really no need for two displays, though. With the indicators, one display can show two or more quantities, alternating (as you have probably seen on exercise bikes).
alternating the displays is a good idea... I've been looking at ways to keep the cost down.

If I have to rewrite firmware, how complicated is that? I can code, some, but not firmware. the closest I've come to firmware is rooting android devices.

On a related question, if I went with a larger display such as this one:

what else would I need to drive it?
The "firmware" is written in C/C++, as is standard for Arduino, and is quite complicated. If you have little or no experience with C/C++ or with Arduino, it would be an enormous challenge to rewrite it.

A standard Arduino can use that display, and it would be rather simple to write a program to count steps and display various quantities. Consider using a $3 Pro Mini from eBay.

The larger display requires additional electronics to interface with an Arduino.