If you can temporarily move the data off of your 2nd drive, you can do this to get what you need:
As instructed above, go to Administrative Tools | Computer Management | Disk Management. right click on the text to the left of the graphic bar of the boot drive (in your case probably "Disk 0" with the word "Basic" below it. Select "Convert to Dynamic Disk. If you're running a basic Windows installation you shouldn't have a problem, if you're running muliple operating systems then this won't work. I'm pretty sure your drives need to be formatted as NTFS (rather than FAT32). If you are already formatted as FAT32, you can use a utility like "Partition Magic" to convert your FAT32 drives to NTFS without losing any of your data. Converting to dynamic disks does not affect your data. But to make this entire process work, I'm pretty sure your second drive will need to be empty until you're finished.
Once you've converted the first disk, do the same on the second disk (Disk 1). You now have TWO "dynamic" disks (which at this moment is no different than haveing 2 basic diks). Right click the first drive and select "New Volume". You now want to create a "Spanned Volume". This will (as far as your computer is concerned) turn your 2 350GB drives into a singe 700GB drive. The wizard will walk you through it - you'll just be adding the 350GB from your second drive to the first drive.
Now you don't have to mess with separate directories or creating new shares, you now have the equivalent of a single 700GB drive that any of your existing directories can use (as well as any new directories you wish to create) Your Roku will also see this just as a single share (oir as many shares as you wish to create). If there is a downside, it's that if one drive fails, then you lose the data on both - but backing up your important data will protect you just as it does now. You can span as many as 32 drives so adding additional drives to your spanned volume is easy. This is the most basic of RAID configurations known as JBOD (Just a Bunch Of Disks). If you find you need additional drives in the future, you might want to check in an expansion card that supports "RAID 5". You can get a card for under $100, and with one extra drive you can get the most of your storage AND have have your data protected from a drive failure. The inexpensive cards are quite slow when writing data, but reading speed is just fine.
I hope this helps, good luck.