Windows only: Mount more than one Gmail account as a remote disk drive with free utility Gmail Drive Config. It works with Gmail Drive, which we've covered in the past.
Yes, it's all very experimental, I did encounter some annoyances, and
Google could probably make it all go poof with a few keystrokes from
some command line prompt in Mountain View. Rest assured, you'll always
be able to access any files added via mounted drives through Gmail
proper. But I got two accounts set up and working as network storage in
just a few minutes. Here's how.

Install Gmail Drive, if you haven't already. Check out our Advanced file and attachment management with Gmail
post for details. Your Gmail account will show up as a mounted drive
under My Computer in Windows Explorer, categorized as "Other" below any
Network Drives.

Set up another Gmail account, if you haven't already. I signed out of
my main Gmail account, and clicked Sign up for Gmail under the login
box. I simply used my current username plus ".files" to create a Gmail
account specifically for file storage. Be sure to set an alternate
email address so you get a copy of your login details just in case.

Now download Gmail Drive Config from Convivea. It's a simple executable, so just click "Run" from the browser or double-click the gmail_drive file in your downloads folder.

It should show the initial drive you set up. Simply click Add Drive to
add another by entering a new name for the drive and a different Gmail
address.

The new drive will also appear under My Computer in Windows Explorer.
Double-click it to enter the password. Rinse and repeat to add up 999
Gmail accounts.

Note: I tried to add a Gmail account for a Google Apps for Your Domain
address with no luck. Not only did it not work, but even after much
fiddling, I couldn't properly Edit or Remove the entry from the list of
accounts in Gmail Drive Config. Also, when trying to remove an account,
I was prompted with a dialog and system beep to close all instances of
Explorer. Not a dealbreaker, just a little annoying. But at the current
price, who's complaining? Gmail Drive Config is a free download for
Windows.