You might run into a few weird issues.
This could overload the camera and cause frame rate to drop or other quality issues. This is especially a concern with low end cameras and speciality cameras (like the Arecont multi-imagers that are processor constrained to begin with).
Additionally, if 2 VMSes are simultaneously connected to a camera, they might change low level settings of the camera that could impact the other VMS.
This comes up often so I'll chime in. In the good old days of analog, your only concern was properly terminating.
Let's talk about IP because the assumption is if it can provide multple streams then it should be no problem. Not a good assumption.
If you pull RTSP video streams from the camera, and it's capable of it there should be no problem but you will have no control over the camera. This doesn't work well for PTZ's.
If you connect via ONVIF / SDK / API / CGI then you will likely have an issue because MOST (not all) VMS or NVR products actually query the camera at the time you add it and after that. They will typically load the values in their database for the camera at that time. A prime example would be NVR/DVR's that use NTP for time regulation and update the camera routinely.
Anyway....you have VMS #1 setting parameters at 30IPS - 1080p - 6MBPS using the CGI or SDK commands.
Meanwhile VMS #2 is connecting using ONVIF Profile S and sets the camera at 15IPS - 720p - 8.1MBPS and happens to not time sync correctly.
You end up trying to level a 3 legged stool if you know what I mean.
I'm not saying there isn't a way, it's just not as easy as it seems.
But hard to lwvel...... Some deep integrations change settings that are not normally visible to the end user / tech / web page. Loss of stream, inability to reconnect, inability to view or find recorded video, incompatible encoding parameters because one assumes the default is set, secondary or tertiary stream changes unintendedly and a host of other ghosts. For a local demo it wouldn't be uncommon to have one system connected using ONVIF or the CGi while streaming a second using just RTSP. Other than a lack of control and programming in most cases it would work. An exception would be any camera using client side dewarp I would imagine. Which product gets the camera first could matter. So now you do a camera reset or reboot? More fun than any single tech deserves, especially just before an important demo.
My experience: whith several VMS/NVR, it's most of the time a mess
Most VMS push their settings to cameras: Resolutions, Fps, Compression and sometimes CBR or VBR, but also ... PTZ Home positions...and also Private masking or motion areas
So 2 VMS will try to push their own settings and will spend time to stop, set, restart streams when detecting an other system changed their owns.
Very few brands, like Axis accept that several VMS create their own profiles (with different profils names) , so can remote monitor independant settings from the same cameras.
Let's add that camera also saturate when you ask too many streams, and as most VMS monitor 2 streams per camera (Rec/view)... it's becoming easy to saturate camera CPU
Very few IP cameras got a CPU indicator (Bosch only from what I know at the time being)
Good day. I have tried this many times in the past and it's always a mess. I can't speak for other systems but one easy way to demo VMS is with simulator cameras. That way you can use good recorded clips of camera positions that might be a it more exciting than an office corridor. However not all VMS have this possibility and yes I work for a VMS company.
What are you trying to show with the two vms? If camera control is not part of your need, and your vms supports it, just use an rtsp url for the video stream only without control. Again, if the camera can support two full rate/resolution streams you should be good. And then there is multicast...if supported.
We are seeing and more need for multicast viewing. Particularly in traffic where the video is shared with many users. VLC as a client.
Just some simple thoughts
In theory, you can use VLC to act as a proxy: receive the stream from the camera (rtsp only) and serve it to multiple VMSes. In the VMS, you gonna need to use the "RTSP Generic driver", if available. Note that you won't be able to use PTZ this way.
yo, neither local dewarping, neither dry contact, audio out... all TCP unicast commands