hi there. consulted the internet after running into a similar need and doing a simple splice of matching colors, and found this, pretty much the only relevant thread anywhere. thought it would be okay to resurrect if i had something to contribute
a brief description of the performance of the spliced outputs for anyone interested: plugging one or the other output into the standard original nes console produced normal behavior. plugging both outputs into the same console also produced normal behavior, rather than the double-tapping i was naively expecting. starting with only one output plugged into a powered-on system, plugging the second output into a second, powered-off console locked the controller (i believe it froze as if down was permanently pressed) and lit up the power LED on the second console; unplugging that second output from the second console would then restore normal behavior. plugging both outputs into separate powered-off consoles and then turning them on simultaneously produced hilarious and bizarre glitches, usually with one, character-sized symbol wallpapered across the screen.
anyway, i'd expected the separate power supplies to be an issue, but now i see that it is not as simple as removing a superfluous +5 VDC connection. the suggestion nitro made about adding another chip seemed rather clever. i have the capacity to perform that experiment, but was wondering if anyone would be willing to verify the theory and confirm the pin numbers given before i did all that soldering. the solution sounded good to me, and if anyone can second it, i'll be happy to try it out and post the results here. thanks very much.