You just need to make a bracket that's 15.2 degrees apart.. Last one I made was a simple piece of pipe about 3 inches long the same diameter of the LNB clamp, I then riveted a cross piece with slots cut in each end, I then used a couple of jubilee clips on some countersunk bolts to act as LNB clamps and used the bolts in the slots to move the LNB's in/out until I had the correct separation.
If you use a quad LNB on the 28.2 then there's no reason as to why you couldn't set up a simple 2 way disecq switch to the FTA box so that it'll pick up both 13 and 28.2 and just feed two 28.2 feeds to the sky box.
If you want to buy the brackets then try an ebay search of "multi lnb bracket" of which they have upto 6 way brackets