In javascript you can install handlers using a message parameters:
Message.install(func,"user.register",90,"param_name","param_value");
This will install a handler for user.register with priority 90.
The handler will be called only if the message will carry a parameter 'param_name' with value 'param_value'.
You can run 2 scripts, the first may install the handler for, let's say, a parameter instance=1 and the second for instance=2.
From outside, maybe regexroute or another java script you can install a handler for user.register to be handled before the ones with filters.
There you can rotate the instance parameter to distribute the handling.