Setting up OctoPrint on a Raspberry Pi for multiple printers

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Running multiple instances of OctoPrint on one Pi is not officially supported. Please refer to this FAQ entry:

Multiple printers per server megathread

Just a link for now to a guide that works
Setting up OctoPrint on a Raspberry Pi for multiple printers

A couple of notes from my own experience following this guide.
Adding a second webcam
99-usb.rules:
If you have two of the exact same webcams the filtering doesn't work if your camera doesn't have a serial number, mine didn't.
Workaround: Use a USB hub, on the hub I used each port had its own identifier under the KERNALS tag
KERNELS=="1-1.2.1" and KERNELS=="1-1.2.2" for my hub was port 1 and 2

/root/bin/webcamd and webcamd2
I recommend using 8081 for webcamd and 8082 for webcamd2

Also I needed to comment out these lines in webcamd and webcamd2

if [ -e "/boot/octopi.txt" ]; then
    source "/boot/octopi.txt"
fi

so like this

#if [ -e "/boot/octopi.txt" ]; then
#    source "/boot/octopi.txt"
#fi

/etc/haproxy/haproxy.cfg
Because of the above change, the haproxy needs to be a little different. Notice the ports in the backend.
This is mine as an example

global
        maxconn 4096
        user haproxy
        group haproxy
        tune.ssl.default-dh-param 1024
        log 127.0.0.1 local1 debug

defaults
        log     global
        mode    http
        option  httplog
        option  dontlognull
        retries 3
        option redispatch
        option http-server-close
        option forwardfor
        maxconn 2000
        timeout connect 5s
        timeout client  15min
        timeout server  15min

frontend public
        bind *:80
        option forwardfor except 127.0.0.1
        acl host_CR10-A hdr_beg(host) -i CR10-A
        acl host_CR10-B hdr_beg(host) -i CR10-B
        use_backend webcam if { path_beg /webcam/ } host_CR10-A
        use_backend webcam2 if { path_beg /webcam/ } host_CR10-B
        use_backend octoprint if host_CR10-A
        use_backend octoprint2 if host_CR10-B
        default_backend octoprint
        errorfile 503 /etc/haproxy/errors/503-no-octoprint.http

backend octoprint
        reqrep ^([^\ :]*)\ /(.*) \1\ /\2
        reqadd X-Scheme:\ https if { ssl_fc }
        option forwardfor
        server octoprint1 127.0.0.1:5000

backend octoprint2
         reqrep ^([^\ :]*)\ /(.*) \1\ /\2
         reqadd X-Scheme:\ https if { ssl_fc }
         option forwardfor
         server octoprint2 127.0.0.1:5001

backend webcam
        reqrep ^([^\ :]*)\ /webcam/(.*)     \1\ /\2
        server webcam1  127.0.0.1:8081

backend webcam2
        reqrep ^([^\ :]*)\ /webcam/(.*)     \1\ /\2
        server webcam2  127.0.0.1:8082
        errorfile 503 /etc/haproxy/errors/503-no-webcam.http

And for accessing outside the network because of how I setup my haproxy I only need one port open, 81

I am using my domain, with an a record to point to my home ip address

The haproxy rules work like this for me
http://cr10-a.mydomainname.com:81 takes me to backend octoprint, my first printer
http://cr10-b.mydomainname.com:81 takes me to backend octoprint2, my second printer
http://cr10-a.mydomainname.com:81/webcam/ takes me to backend webcam, my first printers webcam
http://cr10-b.mydomainname.com:81/webcam/ takes me to backend webcam2, my second printers webcam
http://cr10-a.mydomainname.com:81/webcam/?action=stream takes me to backend webcamstream, my first printers webcam stream
http://cr10-b.mydomainname.com:81/webcam/?action=stream takes me to backend webcamstream2, my second printers webcam stream

That's about it.