Actually, after reading about this closer, it doesn't look like this is possible yet. From this page: http://search.cpan.org/~preaction/Mercury-0.012/lib/Mojolicious/Plugin/Mercury.pm

NOTE: Since Mercury does not yet have a way for brokers to share messages, you must run Mercury as a single process. You cannot run Mercury under Hypnotoad like other Mojolicious applications, nor prefork or other multi-processing schemes. See https://github.com/preaction/Mercury/issues/35 to track the clustering feature.

So, I guess for now the solution is to run under Morbo until this is resolved either by Mercury or Mojolicious.