Search a middle on distance between two bones

Support Animation and Rigging
1

I have a 3 bones: X1, X2 and Y. I need find middle between X1 and X2, and placing at this point Y bone. Sample: X1—Y—X2, after moving: X1———Y———X2. I used IK stretch, but it reduces bone, instead enlarging, and increases instead decreasing :frowning:

2

Give Y a world space copy position constraint targeting X1, at full influence, then give it another targeting X2, at 0.5 influence.