The current situation sort of outlaws clusters automatically because, to fight heavyweight flippers, you have to be about 70+kg (based on the feedback Ive been getting), so two middles as a cluster would only be able to enter on the same terms as a middle on its own anyway.
Also, as I understand the cluster rule, the machines must enter the arena as one machine and then split so you couldnt just enter two in a tag-team style. However, this is a very easy prob to solve (Typhoon Twins using string and paper anyone? Genius that).