This isn't possible, if I understand correctly you want the middle joint to be parent to both the left and right branch of the Y. Joints can only have one parent, using constraints will allow you to get a belnded result between both branches of Y.
Lets say you joints all orientated correctly:
joinYLeft, jointYRight, jointYMid, jointYend.
Duplicate JointYmid and name it jointYmidCon
Parent jointYend to jointYmid and then parent jointYmid to jointYmidCon
Then select jointYright and jointYLeft and then the child jointYmidCon and do a point / parent constrain depending on what kind of result you want (you will want to maintain offesets on the constraint settings).
The result will be the lower half of the Y blending between the positions / rotations of the two top branches.
FX supervisor - double negative