There is nothing in the rules to prevent you from doing a PPL in a twin however; there are a number of issues that might make it unachievable in practice. The cost has already been mentioned and the requirement for 70 hours PIC before you can be issued with a MEP class rating. There is nothing to prevent you from obtaining that 70 hours PIC in a twin as a solo student, except of course insurance; will anyone want to insure a solo student for 70 hours solo in such an aircraft? It has been done in the past and may well be easier in other countries. I have not heard of anyone doing it since the JAA became a reality 11 years ago.