From what I've read, the inability to connect a mini DisplayPort device to an Apple Thunderbolt Display is because of a bug in the Apple Thunderbolt Display. If you connect another device in between the Thunderbolt Display and the Cinema Display, they both should work along with your MacBook Pro's internal screen.
Yes, you can add another monitor if you use a USB video adaptor. No, there are no USB video adaptors that support dual link DVI monitors such as the 30" Cinema Display.
Many USB video adaptors will support 1920x1200, some a little bit higher.
However, you will find the USB video to be slow and annoying if you are used to high performance video.
I suggest you consider using two Apple Thunderbolt displays which are a bit smaller and a bit lower resolution at 2560-by-1440, but you can connect two of them, and you'll actually find as good as your cinema display is, technology has improved noticeably since its release, and the thunderbolt displays are absolutely stunning.
You might be able to connect one thunderbolt display, then attach the thunderbolt-DVI adaptor to that display and attach your cinema display.
To that end, you might find some of the information in this question useful:
How to connect two non-Thunderbolt displays to the Macbook Pro 2011 via Thunderbolt?
Best Answer
Your particular model of MacBook Pro was equipped with an AMD Radeon HD 6750M dedicated graphics card and on-board Intel HD 3000 graphics card. In terms of VRAM you have 1 GB available on the AMD and 384 MB on the Intel (shared with main memory).
This combination is capable of up to 2560 x 1600 pixels on external displays that do not require a dual-link DVI connection.
If you're finding that the highest available resolution is 1280 x 800 instead of 2560 x 1600, this is likely due to you using a single-link DVI connection on a display that requires a dual-link DVI connection. Unfortunately, the 30" Apple Cinema Display is one such monitor.
You would need to connect this display to a dual-link DVI port to access the higher resolutions available on the 30" display. However, your model MacBook Pro does not provide a dual-link DVI connection without an appropriate adaptor and that is why it's not supporting the 30" Apple Cinema Display at its native 2560 x 1600 resolution.
For best results you will need to use a Mini DisplayPort to Dual-Link DVI adapter.
You should refer to this info on Apple's website. Also of interest may be this Mini DisplayPort adapter FAQs page.