Connecting GG input to SMS controller port (how to isolate?)

[cr4zymanz0r]

So basically I'm trying to integrate a Game Gear into a Master System, but have both PCBs for maximum compatibility. Testing was going great until I tried hooking the GG control inputs to the SMS controller port. Here's the current situation:

Test setup:
Pad hacked the GG with each input connected to the corresponding SMS controller port pins (pin 1 for up, pin 2 for down, etc.), and GG start button going to pin 7 (only used by the light gun).
No power cord connected to the SMS at all.
No controller plugged in.
Used Sonic 1 as my test game.

Results:
With start button connected to pin 7: Start appears to be held down or being 'pressed' very rapidly. It quickly goes in-game and then pauses and never unpauses.
With start button not connected to SMS at all: Game boots and goes to title screen normally, but when demo mode starts it gets cancelled out of. If I manually activate start on the GG to get in game Sonic jumps to the right, rolls back down the hill, and then stays crouched down once he stops rolling (then an enemy comes up and kills him a few seconds later). Same exact movement every time, same results with the GG inputs connected to either controller port 1 pins or controller port 2 pins.


Now I got in touch with someone who already integrated a GG into a SMS to ask him how he got around it. However, I found out he simply cut the traces for SMS controller port 2 and used it. That's a no go for me since I don't want to wreck any SMS functionality. I'd also like to avoid modding in another DB9 port as a dedicated GG controller port if at all possible.

I assume some IC or components in the SMS are interfering with the GG inputs, but I had hoped that wouldn't be the case if the SMS PCB had no power. Does anyone have any on how I could keep the SMS PCB from interfering with the GG inputs while not affecting SMS functionality? If there's some simple components or a circuit I could add in that would be great.