GNU Parallel - Multiple arguments

Using GNU parallel, I am trying to run a sub-sampling script that inputs two files and outputs a specific subsampled file. I am using this command:

parallel -j+0 --eta python sub_sample_.2.py ::: file1 file2 ::: file3 file4 ::: file5 file6

But there's no ETA on the command line, i.e.:

Computer:jobs running/jobs completed/%of started jobs/Average seconds to complete
ETA: 0s Left 8 AVG:0.00s local:8/0/1005/0.0

Also only the first four files are processed, but not the last two: file5 and file6.