Interquine - Two programs that output each other in a loop
CJam, 13 + 13 = 26 bytes

{sYZe\"_~"}_~


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Outputs

{sZYe\"_~"}_~


Explanation

{       e# Standard quine framework, leaves a copy of the block on the stack
        e# for the block itself to process.
  s     e# Stringify the block.
  YZe\  e# Swap the characters at indices 2 and 3, which are Y and Z themselves.
  "_~"  e# Push the "_~" to complete the quine.
}_~


Since e\ is commutative in its second and third operand, the other program does exactly the same, swapping Z and Y back into their original order.