I'm not sure if I mentioned this before, so forgive me if I have.
I was wondering if it's possible to hack certain NES games to make them compatible with the Playchoice-10, and with a few alterations to the game. For this example, we'll use TMNT2.
Suppose you hack the game so that it only starts if you insert a coin. If you insert a coin for P1, you play as P1. If you do that for P2, you play as P2. In gameplay, the unused half of the status bar will flicker the message "Insert Coin", and when the coin is entered, that screen is turned into a miniature character select screen. When you lose all your lives, your half of the status screen will show the message "Continue" with the numbers counting down, while the action continues. When both players fail to continue, it cuts to the Game Over screen and takes you back to the title. Also, during the attract mode, "Insert Coin" will be displayed at both portions of the status screen.
It's a bit of work, and TMNT2 itself was ported from an arcade game, but is it at all possible to do something like this?
Interesting. A quick scan says the Playchoice-10 was basically a hardware compatible NES with a bit extra.
Are you planning to do this on an actual Playchoice-10, a replica affair (or a fancy NES mod even) where you can change a few things or an emulator type thing?
The issue of how the coins will work comes into play. There seem to be three main types.
1) Locks the controls until a coin is put in. This is more common in timer driven affairs (I was spared most of these, I can see hating them though even if games were typically hard enough to be functionally the same) which does seem to be the Playchoice-10's method of choice.
2) The coin mechanism acts as an extra button, a hidden button from the arcade (it may be that select does not exist on the front panel) or perhaps some impossible button combo (left and right at the same time for instance). Extra buttons over the original vs impossible combo might change things a tiny bit though if memory serves the NES controller was a saturated shift register so I am not expecting extra buttons.
3) The coin mechanism is the frontend to some extra hardware that either fiddles with the memory or is some bit of IO in there somewhere which the game can read.
3) and 1) can be combined a bit as well and the lives counter in a game might well spit something out and lock the controls.
NES games did allow for a bit of IO too and as you should only be throwing around a few bits at a time at rates nowhere near those which will trouble anything, TMNT2 uses MMC3/TLROM if http://wiki.nesdev.com/w/index.php/Nesdev_Wiki
is correct (and it is one of the better resources for this sort of thing).
On real hardware though it looks like there were some extras pertaining to exactly this.
Each method would impose different restrictions and assuming you are not going full emulation you may instead find yourself getting hands on with one of the programmable chips that people so enjoy playing with these days.
If going for the full arcade effect the "countdown to game over" thing they so often enjoyed might take some more thought to replicate.
Anyway the game over stuff is likely to be little more than a variation on infinite lives cheats, the other stuff is likely text/graphics hacking with the fun coming in how you might speak to the IO. Any harder things might be removing password screens.
If you are doing the emulator bit then if the emulator speaks lua (and many do) you could probably knock this out (or at least as far as the graphics side of things) over an afternoon including the time it takes you to learn enough lua to get by.
In short I do not see a major problem with this or even anything likely to really trouble you, give or take how you might feel about soldering and a bit of light electronics. Given you probably know far more about the Playchoice-10 than I then you are probably already doing prepping to play electrical engineer anyway to sort the expansion card issue.