This web site shows how to make the HST so that you have the same fabric on two sides and different ones on the opposite sides (see option 2 for the easier way). In your case, one of the squares that you use for the first HST will be a 4-patch of the two fabrics you want in that 1/4 section of the block. At least I think that's the way it should work.
http://www.connectingthreads.com/tut...tars__D11.html
Edited - this way there are no y-seams, but I haven't made it so I don't know how difficult it is to line everything up. I think I will try it later...