When two monitors are loaded the MaxScale installation the MaxScale does not accept any queries. The clients get ERROR 1045, failed to create a new session.
The documentation is slightly off on this point in the configuration guide: https://github.com/mariadb-corporation/MaxScale/blob/master/Documentation/Getting-Started/Configuration-Guide.md#monitor
As they are in 1 code block the mistake can be made to copy/paste the whole of it.
I don't know if you consider this a bug or user error (why would the async replication need to be monitored on a galera cluster?) but I thought to report it as it caused some confiusion.
If the issue is related to having any 2 monitors I think it should be fixed as you may want to monitor a cluster with two different plugins.