SQL Server second transaction log seems to be used as well - not only after first one is full
The Log files are used like if it was only one file. Both files have VLF so let's say the first log file have VLF1 to VLF10 and the second one have VLF11 to VLF15. It will write down stuff in the VLF1 and once that one is full, will continue in VLF2, etc... once it reach VLF15, it will then go back to VLF1 (not before) so it's 100% normal that both files still get used.
If you want to get rid of your second file, you'll have to take log backup until the &quot;active&quot; VLF is on the file #1 and then, you'll be able to delete the file.