Here's how I'd do this:
o Mount both drives on the computer.
o Fire up Lightroom and create a new catalog.
o Choose import and select the first drive to import. Let it crunch away, go get a coffee
o Repeat for the second drive. Now all images in both drives are in the Lightroom catalog where you can manipulate them.
o make sure your third drive is mounted
o in the left sidebar, click the "+" next to folders.
o in the menu it brings up, choose "Add Folder" and select or create a folder on that new drive. This is where you'll put everything you want to keep.
o Now you can use Lightroom to browse both of the old disks and drag copy and organize the images any way you want. When you're done, the images you want to keep will have been moved into the folder on your third drive.
o Now, unmount those first two (now depopulated) drives
o Quit Lightroom (if you haven't) and trash the old catalog.
o Fire up Lightroom and either create a new catalog, or open up your existing one.
o Now you can use that "Add folder" above to attach that folder to Lightroom, or Import it through the import function. If you "Add Folder", you should right-click on the folder and choose "Synchronize" to pull all of the images into the catalog.
(one note: don't muck around with editing, keywords, etc in that catalog you're throwing away unless you make sure you flush XMP data before exiting Lightroom, or have the "automatically write meta data to the image" preference option turned on, or you'll lose it when you trash that temporary catalog)
I prefer the temporary catalog for this over doing it all in your main catalog and using the optimize because it guarantees a clean catalog without stuff that optimize might leave behind.
It's a fairly straightforward operation if you're careful. I've done this a couple of times.
(another option: just copy all of the image from both drives into the new drive, and then you can do the purging and optimization later. Depends on whether you want to do that all at once, or over time. Frankly, I'd probably do the latter, because this can turn into a grind and you can find yourself making mistakes or taking short cuts you'll later regret. If you turn it into something you can spend 30 minutes on here or there, it'll be a less painful process....)(