We could run multiple instances of FastNetMon on same server

First of all, please create multiple configuration files for each instance.

In each file you should specify unique paths for following options for each separate instance:

# Path to pid file for checking process liveness 
pid_path = /var/run/fastnetmon.pid

# Path to file where we store information for fastnetmon_client
cli_stats_file_path = /tmp/fastnetmon.dat

Next you need to specify custom path to configuration file:

./fastnetmon --configuration_file=/etc/fastnetmon_mayflower.conf --daemonize
./fastnetmon --configuration_file=/etc/fastnetmon_hetzner.conf --daemonize

FastNetMon CLI client expects stats file with default path /tmp/fastnetmon.dat so you need to set custom path with environment variable:

cli_stats_file_path=/tmp/fastnetmon_second_instance.dat  ./fastnetmon_client