When the new log files were added, SQL Server immediately filled them to match the fullness percentage of the original log files. As I manually increased the size of the log file, SQL Server again shifted the transactions around to keep both log files at the exact same fullness percentage. So, if the first log was 95% full, the second log was 95% full, regardless of the actual log file sizes. If the first log was 80% full, so was the second. The more I expanded the second log, the more transactions SQL Server would move to it, always keeping the fullness percentage the same on both log files. The larger the second log became, the more space it freed up on the first log, but the 2 logs were always exactly in sync in terms of fullness.
This is some interesting behavior, especially because transaction log files don’t use proportional fill.