User blog comment:Xilinoc/Ch. 583 - The Headless Star 2/@comment-26420452-20140604083601/@comment-1338810-20140604164538

I figure the cross is both a Quincy homage (for his mother's side) and a nod to the kanji for "ten" (for his dad's side, being that Isshin was captain of the 10th Division), on top of being an obvious dual wielding attack form.