Using 2 displays on late 2011 Macbook Pro I am running a late 2011 MacBook Pro (Processor  2.4 GHz Intel Core i5).
I have 2 external monitors that I would like to use with my music production software (ableton live).

I know that I can connect one monitor via thunderbolt to VGA or DVI and a second via USB. However I am conserving my USB slots and already like to run a multi HUB from one of my two USB slots, with my external sound card running from the second slot. My question therefore is:

Would the power draw/data be too much to power a monitor as well as say, a MIDI keyboard and a mouse from an active USB hub. (All three devices are plugged into the hub and therefore into the same USB port on the laptop itself).
Officially your MacBook Pro is only capable of simultaneously outputting a video feed to the internal display and a single external monitor. 

However, there are a number of USB display adapters that will allow you to support a second external display.

USB Display Adapters 

These are USB to VGA or USB to DVI adapters. Some of these support up to a maximum resolution of 1920x1200 or 2048x1152.

These three are guaranteed to work on Apple Macs and support a maximum resolution up to 2048x1152:


Kensington Multi Display Adapter USB 3.0
IOGear USB 3.0 to DVI External Video Card
OWC USB to DVI/HDMI/VGA Adapter 


These adapters work in by compressing and decompressing the video signal in order to utilise the limited bandwidth provided by USB 2.0 or USB 3.0. 

However, you should be aware that there are limitations to USB Display Adapters. Namely they do not support OpenGL 3D hardware acceleration and you are likely to notice a lag.

If you're intention was to use these for gaming or animation/video editing, I'd say forget about it.  But with your music production software it may be fine as the emphasis will be on the audio side of things, especially if you keep your graphics intensive work limited to the other display connected via the Thunderbolt port.

As far as how well a hub will go trying to connect to another display as well as your MIDI keyboard and a mouse, your success will depend on the quality of the hub and ensuring that it's also got its own power source.

A better option may be to also get a hub that connects to your Thunderbolt port that connects the display and also provides an additional USB port for example.