[Balimbanana]

Plugin A:

public void OnPluginStart()
{
	Handle cvar = FindConVar("sm_stoprecord");
	if (cvar == INVALID_HANDLE) cvar = CreateConVar("sm_stoprecord", "0", "Description", _, true, 0.0, true, 1.0);
	CloseHandle(cvar);
}

public void OnMapEnd()
{
	Handle cvar = FindConVar("sm_stoprecord");
	if (cvar != INVALID_HANDLE) SetConVarInt(cvar,1,false,false);
	CloseHandle(cvar);
}

Plugin B:

public void OnAllPluginsLoaded()
{
	Handle cvar = FindConVar("sm_stoprecord");
	if (cvar != INVALID_HANDLE) HookConVarChange(cvar,StopRecordChanged);
	CloseHandle(cvar);
}

public void StopRecordChanged(Handle convar, const char[] oldValue, const char[] newValue)
{
	if (StringToInt(newValue) == 1)
	{
		//stuff
	}
	else
	{
		//stuff
	}
}

You could also set the CVar in plugin A to be a global handle, but it isn't really necessary.