Bad connection on the player 2 display select signal wire. It's the yellow-red wire from MPU connector J1 pin 21, to pin 15 at the player 2 display.

You need to do a continuity test.
Grab your multi-meter and set it to resistance ohms Ω mode. If your meter isn't auto-ranging, set it to 200k ohms range.
With the game OFF, put one meter probe on the Alltek MPU board at pin 7 of the U12 chip, put the other meter probe on the player 2 display board at pin 1 of the 16 pin chip. The meter reading should be about 21k ohms.
You can confirm it against the reading for the respective control signal going to display 1. MPU U12 pin 5 to player 1 display board chip pin 1. Again should be around 21k ohms.

Note the signal controlling the player 2 display is also used as part of the games switch matrix. Another common cause of this issue is if that signal on the switch matrix is shorted to ground and this usually happens when someone has hacked the wires on the coin/slam switches on the coin door specifically the red-green wire. The easy way to see if this is the cause is to disconnect the J3 connector from the MPU board which is the coin door switch harness. If the display problem goes away, inspect the red-green wire on the coin door for a short to the metal frame which is grounded.

When the game code periodically refreshes the displays, it first updates the player 1 display and ends with updating the credit/ball in play display.
Your symptom is indicating that the signal controlling that player 2 display isn't being seen properly resulting in the player 2 display accepting any data being sent to any display and because the credit/ball in play display is the last written, that's why you're seeing that info mirrored on the player 2 display.
I hope this makes sense.

The MPU LED should go dim after the 7 flashes which is an indication of normal operation that the CPU is running.

Maybe you were missing the mylar insulator between the slam switch and metal coin door. See the half moon mylar insulator piece stuck on the door from factory below: When the mylar is missing/removed, the red-green wire attached to the weighted leaf shorts against the door which is grounded.
It could also be the O.P's problem here too.

IMG_0014a.jpg

Chips with pins on either side are known as Dual InLine Package (DIP).
Pin 1 may have a dot/divot next to it and/or there will be a notch on the thin package end next to pin 1. Pins are numbered anti-clockwise.
See these examples:
Pin_Numbering1.jpg
Pin_Numbering2.jpg
MC14543.jpg
Pin_Numbering4.jpg