Moneyshot, the HV curve for twins is in the event of a single failure not double. I think you'd be hard pushed to find info on the engineless HV curve for any twin.
What I find interesting is the different philosophies shown by manufacturers in developing the landing profiles. Some like Bell and Eurocopter go for the keep the speed up until approaching ground effect, then slow down; fast /shallow. Sikorsky on the other hand hvae gone for the fast steep approach with high rate of descent followed by split a**e flare to recover at the bottom: practically an engine assisted recovery from autorotation. (that's the UK Group A landing anyway, LDP is 50 kts at 100 ft with 750 ft/min max, keep the 50 kts to 50 ft then flare with up to 20 degrees nose up and apply power to come to a hover). Both work and both keep you away from any HV curve.
The following notes from AC-29C (advisory circular that amplifies FAR-29: large rotorcraft certification) should dispel any rumours about HV curves not being applicable:
Page B - 71
(B) Landing distance. Approach and landing path requirements are stated in general terms in paragraphs (b)(2) and (4) of § 29.75.
The approach path must allow smooth transition for one engine inoperative landing and for balked landing maneuvers and must allow adequate clearance from potentially hazardous HV combinations.
Paragraph (b)(4)(ii) implies that a less restrictive HV envelope may exist for the Category A approach condition in comparison to that determined under high power conditions in § 29.79.
The manufacturer may elect to use this added capability. The added capability arises from the fact that lower power levels, a lower collective setting, and an established rate of descent accompany typical approach conditions as opposed to the more critical high power conditions of § 29.79.
Landing distance is measured from a point 50 feet (25 feet for VTOL) above the landing surface to a stop. For flight manual purposes, the distance is from the point at which the lowest part of the rotorcraft first reaches 50 feet (25 for VTOL) to the foremost point of the rotorcraft (including rotor tip path) after coming to a stop.