Viking, you can't take applications and deliver events to them simultaneously in different threads unless the application is specifically written with this in mind. Some events will change data (e.g. key presses in an editor), others will read data (e.g. redrawing text), and unless threads are synchronised with semaphores or mutexs to prevent reading when the data is in mid update and inconsistent, the application will fall over.
As for Hydra, adding it to a Risc PC would have done precisely nothing. To take any advantage of it, applications would have to be extensively re-written, and with the RISC OS still being CMT, getting any benefit out of the additional cores is vastly more difficult than on any other platform. That's why when the StrongARM came out, offering faster single core operation, it was dropped like a hot brick.