I don't think it can be right to run two engines on one machine. Unless there are no time limits anyway, an engine is entitled to think (i.e. to make full use of the CPU etc.) for all of its own time and for all of its opponent's time.(Incidentally, making the engine do the listening means you can't have more than one engine running on one machine. Getting the referee to do the listening means you could run two engines on one machine. )