Hi,
Per searching, there is no documented information stating the / remoteguard is restricted to 2 simultaneous users but one user shared his experience on "Mstsc.exe /remoteguard works only with administrators. This has been tested in RDS deployments and confirmed with RDS ETL tracing".
Details could be referred to below link:
mstsc /remoteguard doesn't work if connecting to RDS servers
https://github.com/MicrosoftDocs/windows-itpro-docs/issues/1483
Please note: Information posted in the given link is hosted by a third party. Microsoft does not guarantee the accuracy and effectiveness of information.
Hope this helps and please help to accept as Answer if the response is useful.
Thanks,
Jenny