Test Steam sessions on one pc (tried with Sandboxie)

I am using Steam session api for matchmaking in my game, but I am looking for a convenient way to test it using one pc. For example, I want to launch two instances of my game at the same time, so I can create a match on one and join that match from the other.

I tried Sandboxie to launch two Steam clients and two standalone game instances, but it seems that only the first launched correctly connects to Steam client.

I tried this: opening Steam client and launching a standalone instance of my game, then inside Sandboxie opening another Steam client (logged with another account) and another instance of the game. The second instance of the game prints this in the log:

LogOnline:Warning: STEAM: Failed to initialize Steam, this could be due to a Steam server and client running on the same machine. Try running with -NOSTEAM on t
he cmdline to disable.
LogOnline:Display: STEAM: OnlineSubsystemSteam::Shutdown()
LogOnline:Warning: STEAM: Steam API failed to initialize!
LogOnline:Display: STEAM: OnlineSubsystemSteam::Shutdown()
LogOnline: Unable to create OnlineSubsystem module Steam
LogOnline: Unable to load default OnlineSubsystem module Steam, using NULL interface

If I launch the Sandboxie instances first, the game connects correctly to its Steam client, while when I launch the instance outside Sandboxie it gives the same error messages.

Anyone knows a way to test Steam api while running two game instances on the same machine? Now I am using two machines, but it should be quicker to use just one.

did you fix this or find any work around ? im having the same exact problem

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Apparently Steam has code in place specifically to disable using sandboxie because of issues with people boosting in games with it.