You've got 3 options.

1. if your mac mini is an intel based one - install bootcamp and run windows on it.
2. Theres a cross platform piece of software called synergy - there are builds of this that support keybroadcasting - this would not be easy to setup though, its very old now and generally a pain to configure on the mac.
3. Pick up a cheap wireless USB keyboard and 2 receivers and do the broadcasting in hardware. This would probably be the easiest way