To trade two brokers with one Zorro instance, do this (per Petra's recommendation):
https://zorro-project.com/manual/en/brokerarb.htm

To trade two brokers with two independent Zorro instances, no need to duplicate dlls. Just set up different account numbers info in the user field:
https://zorro-project.com/manual/en/mt4plugin.htm

This is also required reading.
https://manual.zorro-project.com/asset.htm