I think you are looking for this:
Event ID 4778 : A user has logged by selecting Switch user command (Fast User Switching).
Event ID 4779 : A user has logged back on using Switch user command (Fast User Switching).
But these events are displayed in the Security event log only if you configure "audit consolidation events" in the audit stratum.

Sorry for the French screen, but I think it's better than nothing.
" ", 4778 4779, - :

, powerShell , , :
clear-Host
$UserProperty = @{n="User";e={$_.ReplacementStrings[0]}}
$TypeProperty = @{n="Action";e={switch($_.EventID) {4778 {"SwitchOn"} 4779{"SwitchOff"}}}}
$TimeProeprty = @{n="Time";e={$_.TimeGenerated}}
Get-EventLog -LogName Security -Source Microsoft-Windows-security-auditing | where {$_.EventID -eq 4778 -or $_.EventID -eq 4779} | select $UserProperty,$TypeProperty,$TimeProeprty
, .
JP
PS: " " .