I can't actually see anything on the DSA website which prohibits you from booking two practical tests but. like Ethel, I wouldn't recommend attempting to do so.
If you do fail your first test, you won't necessarily have to wait as long for your next appointment as Norman suggests (especially if you're able to accept an appointment at short notice due to someone else cancelling):
Quote:
"If you fail the test you should ask the examiner for some feedback to help prepare yourself for your next test. Your driving report form will also show you where you made any mistakes.
You can take another practical test 10 working days after your car or motorcycle test."
Source:
http://www.dsa.gov.uk/Category.asp?cat=104
Chris