I had the 2010 Multi1200 and was one of the first to try the Alt-rider topplate for mounting monokey givi topcase.
I kept the plate when I sold the bike, and it does bolt right up to the grab handles just like I had it mounted on the multi12.
Today was my first ride 2up. My wife was too close to me, because of the mounting of the topcase and backrest. Keep in mind my topplate was a first off from them and I think it has changed since. It was even closer by about 3" and I drilled and moved it back so she could fit on the Multi12, but is at the very back, I can not move it any farther back without adding an extension to the plate.
Another thing, it flexes pretty good bit, don't think it is a problem, just that it flexes more than I like.
I am curious if the DP topplate has more support arms in addition to the 4 top mount points of the grab handles?
Bike handled very well 2up, I had preload on shock adjusted full on.