a crazy idea mine is 2 players for BATMAN RETURNS SNES!!! 
I'd love a 2 players hack but for Batman Returns... on NES. The crazy thing is the game seems to be heavily based on TMNT3, by that I mean it seems obvious to me they looked at how things were done in TMNT3 for heavy "inspiration" and faster dev time; and TMNT3 does have a 2 player mode.
Hello there, I've been trying Zelda II - Adventure of Link 2 player co op.
Whenever there's enemies on screen, the game slows down incredibly. I've seen people on Youtube play it without slowdown. Other games and NES ROMs have no slowdown at all. Any tips?
OS: Windows 10
RAM: 16 GB RAM
CPU: Intel Core i7-7700k CPU @ 4.20 GHz
Tried Retroarch with NEStopia Core
Tried Higan
Did you try overclocking? Like NesDraug said Mesen should be able to do that, so should FCEUX. For instance in FCEUX go to config->timing, click on Overclocking and set both setting to 50 or so. Notice that it says "old PPU" only, some games do not support this overclocking, you'll notice if you see graphical issues. The only games I'd had issues with so far were Natsume games.