Dana between the twin engine Vs single engine kolb what changed in terms of flying surfaces.
They're pretty close. The big difference between the Flyer and the Ultrastar (other than the engines) is structural; the Flyer has non folding wings, two spars, and two struts, whereas the Ultrastar has folding wings with a single large tube spar, and one wing strut.
Powered sailplanes (certificated) are limited to a single engine by the FARs. E-AB is more "fuzzy", of course, but since powered sailplanes are flown with a conventional pilot's certificate and a Glider rating, I can't see how the need for a multi-engine rating wouldn't still apply. Where are you getting that you can have a multi-engine powered sailplane and not need the appropriate multi-engine sign-off?
For airplanes you have ratings for "airplane single engine land" and "airplane multi engine land", but for gliders it's just a "glider" rating, which obviously doesn't come in "single engine" or "multi engine" flavors. Motorgliders require a "self launch"
endorsement; it's not a rating.
For that matter, you don't even (I think) need a glider rating to fly (solo only) an experimental glider, just as a PP-ASEL can legally solo (but not carry passengers in) an experimental seaplane, or a multi engined plane, unless the aircraft's operating limitations specify otherwise.
Dana