<log4net>
  <appender name="InfoFileAppender" type="log4net.Appender.RollingFileAppender">
    <param name="File" value="InfoLog_"/>
    <!--<lockingModel type="log4net.Appender.FileAppender+MinimalLock" />-->
    <param name="AppendToFile" value="true"/>
    <param name="RollingStyle" value="Date"/>
    <param name="DatePattern" value="ddMMyyyy'.log'"/>
    <param name="StaticLogFileName" value="false"/>
    <layout type="log4net.Layout.PatternLayout">
      <conversionPattern value="%date [%thread] %-5level %newline%message%newline%newline"/>
    </layout>
    <filter type="log4net.Filter.LevelRangeFilter">
      <levelMin value="INFO"/>
    </filter>
  </appender>
  <appender name="ErrorFileAppender" type="log4net.Appender.RollingFileAppender">
    <param name="File" value="ErrorLog_"/>
    <param name="AppendToFile" value="true"/>
    <param name="RollingStyle" value="Date"/>
    <param name="DatePattern" value="ddMMyyyy'.log'"/>
    <param name="StaticLogFileName" value="false"/>
    <layout type="log4net.Layout.PatternLayout">
      <conversionPattern value="%date [%thread] %-5level %newline%message%newline%newline"/>
    </layout>
    <filter type="log4net.Filter.LevelRangeFilter">
      <levelMin value="ERROR"/>
      <levelMax value="FATAL"/>
    </filter>
  </appender>
  <logger name="Logger">
    <appender-ref ref="InfoFileAppender"/>
    <appender-ref ref="ErrorFileAppender"/>
  </logger>
</log4net>

public static class Logger
{
    public static ILog Log { get; } = LogManager.GetLogger("Logger");

    public static void InitLogger()
    {
        XmlConfigurator.Configure();
    }
}

// сконфигурировать логирование
Logger.InitLogger();
Logger.Log.Info("Global.asax, Application Start...");
Logger.Log.Error("Global.asax, Application Start...");

<lockingModel type="log4net.Appender.FileAppender+MinimalLock" />