I found an old copy of synergy broadcast on my hard drive. IIRC, I had to build it from the source and i don't think it does mouse broadcasting, but you can seamlessly move the mouse from one computer to the other. Keep in mind that the date this build has on it is from a LONG time ago soooo.... your results may vary. I think the tilde key toggled on and off broadcasting.

I uploaded a zipped copy of the source and my binaries that you can try out. You should be able to find it here https://dl.dropbox.com/u/2110626/synergyB.zip.

If you want a much more friendly experience boxing i would strongly suggest running ISBoxer on one or two computers. I run five accounts on one computer and only one monitor without any issues at all. As MiRai pointed out earlier in this post (or one of your others) I don't think you will find anyone else using this for boxing when there are much cleaner solutions out there. But let us know your results.

Hope this helps