Serilog : Log to different files

You can do this by first making sure the performance counter events are tagged with either a particular property value (OpenMappedContext() in LibLog) or from a particular type/namespace.

var log = LogProvider.For<MyApp.Performance.SomeCounter>()
log.Info(...);

When configuring Serilog, a sub-logger with filter applied can send just the required events to the second file.

Log.Logger = new LoggerConfiguration()
    .WriteTo.Logger(lc => lc
        .Filter.ByExcluding(Matching.FromSource("MyApp.Performance"))
        .WriteTo.File("first.json", new JsonFormatter()))
    .WriteTo.Logger(lc => lc
        .Filter.ByIncludingOnly(Matching.FromSource("MyApp.Performance"))
        .WriteTo.File("second.json", new JsonFormatter()))
    .CreateLogger();