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
Please note: Information posted in the given link is hosted by a third party. Microsoft does not guarantee the accuracy and effectiveness of information.
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