... Not necessarily. One of the advantages of the 2:5 diet is that you are not fasting in the true sense. On fast days you merely restrict calories. Rule of thumb, 600 for men and 500 for women - 1/4 of the avg required.
To The Don. The idea is not to do back to back fast days, but to have at least 1 day in between.
The version of the 5:2 diet being promoted by Michael Mosley (apologies for my earlier misspelling) isn't prescriptive about whether the days should be consecutive or not. Michael Mosley tried both consecutive and non-consecutive fasting days and preferred the latter.
Do the fast days have to be non-consecutive?
The short answer is that it doesn’t matter. Some people prefer doing two days back to back, others prefer to split ie Mondays and Thursdays
http://thefastdiet.co.uk/michael-answers-frequently-asked-questions/