I've seen that several times with Ku on a BUD. What it usually was for me, was that the feedhorn was not at the proper focal point. If the feed is at the proper focal point, the signal pattern will be a single spot, however as the feed moves closer or further, the signal pattern blurs out, until it begins to look like a donut. When my feed was at the wrong focal length, I'd get double reception across the arc on Ku, and yet on C-band, it wouldn't do that, and C-band would generally peak at a different point, in between the 2 Ku peaks. With my C/Ku feed, I eventually had to first locate the satellite using C-band, which would generally end up not giving very good Ku reception, THEN, I would move the feed in/out to peak the Ku, after which Ku would focus as a spot at the same point as C-band.
If you just have a Ku feed, then you can't do what I did, but what I would try would be to move the dish in the middle between the two points where you see the 2 Ku signals, then try to adjust the focal length to get maximum Ku signal.
It's basically just that signal patterns have side lobes, and when the focal distance is off the sidelobes get bigger and the center lobe goes away.