I would save them to different folders and use "Bulk Rename Utility".
You simply right click the Left folder, "Bulk rename here" and:
Do the same with the Right folder, but this time you start with 1 instead of 2 but keep the increment set to "2" (so you'll rename them 1, 3, 5, 7, etc.).
PS: Don't let the complexity of the application scare you. You'll only need one or two features anyway.