Multiple Transaction Log files and performance impact
Transaction log writes are sequential. Only one of the log files will ever be written to at any one time, so having multiple files - in and of itself - can't possibly change your I/O patterns for that database.

Unless you are getting lucky. For example, you've added a second log file to an SSD or otherwise faster or less busy disk, or split the log files across multiple disks and have done so for multiple databases, and you are observing better I/O now because the log has switched to that file on the faster disk, or is more isolated from your other data/log files. In other words, I believe any observed I/O difference is due entirely to other factors and is merely a coincidence, not due to the fact that you added log files alone. SQL Server is explicitly designed to only use one log file at a time - so how could multiple log files possibly improve log write performance, unless the current log file is on faster / more isolated disk? I think you need to provide better empirical evidence (and in doing so you may discover for yourself the true cause of the improved performance).

Please read these posts in full - they were written by a pretty smart guy who worked on the SQL Server storage team for quite some time, so I don't think he's making any of this stuff up for fun:


Multiple log files and why they’re bad
Importance of proper transaction log size management


Also Kimberly Tripp touches on this in a worthwhile article:


8 Steps to better Transaction Log throughput


Note that none of the 8 steps involves adding a transaction log file. In fact she recommends against it.

There are other perils to having multiple log files, particularly if they are large (think RTO) - and there really is nothing to gain.