This possibility came up in my planning thread for a Yorkist TL I've been hashing out for a while, and I couldn't resist exploring it, even if it isn't actually used in the TL.
Anyway, what if Edward IV manages to wed Edward, Prince of Wales with Juana of Castile, daughter (and unexpected heiress) of Ferdinand and Isabella? We'll say Anne of Brittany marries Richard of Shrewsbury just to tie up that loose end. If Edward lives to succeed his father and in-laws, how would this personal union of England and Spain work out? What are the consequences going forward?