From JAR OPS 3 Annex 1 to 3.005(H)
(d) Operating requirements
(1) The Helicopter. During HHO, the
helicopter must be capable of sustaining a
critical power unit failure with the remaining
engine(s) at the appropriate power setting,
without hazard to the suspended
person(s)/cargo, third parties, or property.
(Except for HEMS HHO at a HEMS operating
site where the requirement need not be
applied.)
This would seem to be sensible except for the get-out clause - implying that that a single would be OK at a 'HEMS operating site'