There's a lot to like about the Surface Pro, but we can all agree that its limited screen real estate isn't one of the device's most distinguishing features. Even tablet friendly applications like Manga Studio tend to crowd the Surface Pro screen, leaving precious little room for actual drawing.
Adding a second desktop monitor via the miniDisplayPort is an obvious solution for home or office, but isn't really practical when you're on the go. Korean developer Easy n Light has a fascinating, more portable solution in the form of the iOS and Android apps TwoMon and TwoMonUSB.
Both apps allow you to extend the Windows desktop to a smartphone or tablet via Wifi or USB, respectively. I didn't test the iPad version of TwoMonUSB, but assuming it works like the Android example shown above, it's a great option to consider if you carry around multiple devices.
For the app to work, you need to install a server application on your Windows desktop. Download the app to your Android or iOS device, start it up and connect a USB cable to your pc.
It occasionally takes a little plugging and unplugging of your cable to work, but eventually the software and Windows connect and you'll soon see your desktop extend to the second monitor.
Because the TwoMonUSB app relies on a USB connection, I noticed no lag between the two displays. I assume the same can't be said for the wifi version. Other wireless solutions I've used all exhibited significant lag.
In the specific test I ran with the Surface Pro 2 and the Samsung Galaxy Note Pro 12.2, it was fascinating to see that touch and pen work on both screens. Pressure wasn't recognized on the Note Pro, however. But the idea here isn't to draw on the external monitor; rather the idea is to unclutter your canvas by moving the UI to the second screen. I tested with both Manga Studio and Photoshop CC and it worked great.
It's also unlikely you'd use a second screen that's bigger than your Windows tablet as I show above. I tried to test with a first generation Nexus 7 (my only other Android device) and I couldn't make it work.
UPDATE 4/9/2014: Thanks to the help of reader Reginald Atkins, I just got TwoMonUSB to work on my Asus first-gen Nexus 7. In order for the Windows desktop to control the Android device, the Google USB driver must first be downloaded from here: http://developer.android.com/sdk/win-usb.html. In Device Manager, the tablet will show as mounted but missing the required driver. Point Windows to your newly downloaded .inf file and the tablet will be mounted. It took me a couple of software restarts for the Windows and Android devices to finally see each other. But it works. The disadvantage of using TwoMonUSB on a capacitive device like the Nexus 7 is that you'll need to use your finger or a different stylus for that screen and tiny desktop touch targets aren't easy to hit with fat fingers or pen tips.
Besides testing with the Surface Pro 2, I also extended the display of my Sony Flip 15A onto the Samsung tablet. As with the SP, pressure sensitivity was not available on the Note. It was also weird having to remember to switch between the S-Pen and the Sony Active pen.
One important note: to make the Android app work, you'll need access to the Developer Settings menu on your device. I Googled around for the way to show this hidden menu on my Galaxy Note Pro. In the developer settings, you have to enable USB debugging.
TwoMonUSB isn't cheap as apps go, costing about $9 for each version. The desktop server download is free.
I don't know that I'll actually use any of my machines in this configuration, but it's certainly cool to know that it's possible. And compared to geeky solutions like Roccat's Power-Grid, it's much easier to extend the desktop than program a control grid for your individual application.
Hats off to reader Bob Roach who turned me on to this option. If you get a chance to try TwoMon or know of other similar products, leave a note in the comments section below.