What i'm thinking is not difficult, but hard to explain. Going with your example, you have blocks with Christmas fabric on one side & thanksgiving fabric on the other.
Take 2 blocks and place them right (Christmas) sides together. fold back the backing (Thanksgving) fabric on both blocks and sew just the Christmas fabric. finger press the seam open. Now the 2 blocks are side by side & just the Christmas fabric joined. Press a 1/4" seam on one of the Thanksgiving blocks and lay this over the raw edge of the other block. Whip stitch or machine top stitch. Phew!