Mac Command line render - renders with both GPU AND CPU regardless of render settings

[chronopsis]

I'm trying to render several different cameras in a scene using a command line render script on Mac OS. The scene has two render layers, #1 is set to use GPU only, #2 (an ambient occlusion pass) is set to use CPU only. When I run the script the render works, but it ignores the render settings and uses both GPU and CPU for layer #1. Since I've never had a benefit from hybrid rendering, is there a flag for command line rendering to tell Vray to only use GPU/CPU?