See posts by plecktrum84 below for possible solution. This different tool might require different steps to make splitscreen work. They get changed often so there's no use in giving a guide for them here. Just follow the steps provided by these programs.
It still requires you to have your own copy of the Super Mario 64 ROM. Alternatively you can try djoslin0's own compiler for the coop mod:.Do not ask for warez, in this case copyrighted ROMs, here. You can simply build the game together with the coop mod using sm64PCBuilder2 which needs a legally obtained ROM of the USA version of Super Mario 64.A PC build of Super Mario 64 with the Coop mod by djoslin0.