I suspect you might be able to do this, but you'd have to use more than one multiboxing app.

As an example:
  • Keyclone/ISBoxer/wtfever controlling accounts 1-5, with accounts 6-8 blocked so it doesn't send or capture those clients.
  • Second boxing software such as Octopus where you can disable the keyboard and mouse hooks.
  • Set Octopus to ignore clients 1-5 and only control clients 6-8. With the kb/mouse hooks disabled, it would ignore local input.
  • Run Octopus as a client instead of server so it would take input from the Octopus server instance on the 2nd PC.

Of course, this is all conjecture as I don't have the PCs here to test it. And I'm not aware of a boxing app that supports both mac and PC environments, though you might be able to run Octopus in a Windows VM or something.

Bears some thinking on but it might just be possible.