Well it is possible ! Actually, I know only one game in which we can use multiple keyboard and mouse on the same PC : "Trine 2". We can up to 3 players, each with 1 keyboard and 1 mouse for controlling 1 character. It was a little buggy but it worked. There was up to 3 cursors in the menu !
By default, Windows (and Monogame) treat all the keyboards you plug as one. Same for the mouse : if you plug two mouses (mice ? sorry i'm french ^^), both will control the same cursor. For most applications, it's not useful to have multiple keyboard/mouse, except for the very special case of a splitscreen-PC-game ! Ah ... I miss the time when we could play most console games with splitscreen rather than through network.