Crypt Incursion stack help
When you cast a spell, it goes on the stack before it resolves and takes effect. While it is on the stack, both players can cast instants or activate abilities "in response" to the spell or ability on the stack, and these responses will go on TOP of the stack. Once both players say that they don't wish to do anything else, the spell or ability on the TOP of the stack resolves. This means that the second Crypt Incursion will resolve first, so when the first one resolves, your graveyard will no longer have creatures.

You cannot change which graveyard you wish to target in response. You choose your targets as you cast the spell (as you put it on the stack), and you cannot change that choice later.

From the Basic Rulebook:


  The spell doesn’t resolve (have its effect) right away—it has 
  to wait on the stack. Each player, including you, now gets 
  a chance to cast an instant or activate an activated ability 
  in response. If a player does, that instant or ability goes on 
  the stack on top of what was already waiting there. When 
  all players decline to do anything, the top spell or ability 
  on the stack will resolve.


and 


  You choose the targets for a spell when you cast it, and 
  you choose targets for an activated ability when you 
  activate it. If you can’t meet the targeting requirements, 
  you can’t cast the spell or activate the activated ability. 
  Once you choose targets, you can’t change your mind 
  later.