determining horizontal gene transfer between two genomes?
Since the parametric model is applied to only a single genome to see if there is evidence of recent HGT via a change in the average content in coding regions, it will not tell you from where the HGT came from, just that a HGT likely occurred in an area of the genome. Since you are trying to specifically see if there was a HGT between L. lactis and L. helveticus you would have to run a parametric analysis on both, find a HGT candidate on one of the genomes and then find that sequence as a non-HGT candidate on the other genome and via BLAST.

These papers give a good argument to why to use genome signatures instead of GC content as a parametric method of detecting HGT.

Review from where most of the Wiki figures came from

Genome signature paper

Newer Genome signature paper