The cool thing is that if you setup your input system properly you can actually pretty easily allow different combinations of input devices. I would
not suggest a shared keyboard (single keyboard for 2 players) unless your controls are very simple (only using arrow keys and WASD would be the only "okay" option).
You should read up on the "new" Unity Input system and take a look at the asset store, there are some control systems that even support Playmaker out of the box. Hutong Games even released a Playmaker Tutorial.
You can even explore more creative solutions, like connecting 2 phones as controllers with an on screen UI that makes for a "digital controller" - although that might be tricky to achieve without input lag.
All in all: The first step would be to get the game in a state where you can confidently control all the major systems with a single main input device. For a fighting game this would be the controller. Then you would port that input schema over to a secondary control device, the keyboard in this example.
Then you would start adding code that allows multiple input devices and a per-player choice of said input.
When all is done, then you can start thinking about pricing of your game! :b
