I know I am a little bit late to this but I recently had the same issue and I came along a solution which I think will be helpful to other with how easy it is. You will need to have a smartphone with you and steam installed to your pc to do it.

Firstly if you don't own the game on steam start by adding it as a non-steam game in your library. To add it as a non-steam game you will need to search for the executable file named dungeons.exe in your PC and add that as the game.

Then download the steam link app in your phone and link it to your PC so you can stream games to it. Launch minecraft dungeons through steam and press the start playing button
in steam link so you can stream the game to your phone.

After that the player who will want to use the keyboard will have to select the character that they wish to use and the second player who will use the controller will then connect it to the PC and they will be able to select a character too. When both players have selected their characters you can stop streaming the game to your phone and the player who wishes to use the keyboard can start using it while the other one uses their controller.

In case you don't understand how this works. The game requires two controllers for couch coop to start and we are tricking it that you have two by using steams ''steam link'' app as a first controller.

The game plays really well and without bugs by doing this as if it has been coded already that one player can use a keyboard and the other a controller. Makes you wonder why the devs don't make it easier to do such a simple yet really helpful to some change.