Can someone tell me why if i put two identical servos hs5645s on each elevator and plug one into ele port and other one in aux on dx8 tx one moves faster than the other ?
linkages are identical mechanically

First, isolate the problem to see if there is an issue with the servos speed, or a programming issue in the radio. Just swap the two servo plugs. If the same servo is slower than the other, its an issue with the programming of the servo. If the problem moves to the other servo, then there is probably a programming issue in the radio. On the DX8, each channel can have the servo speed slowed down. Gotta be one or the other.
Good luck,
Pete

I have a number of Hitec Digital servos including the 5645. I also have a DX8.

Hitec Digital servos adjusted, using the Hitec Servo Programmer. There are a number of parameters you can set including Neutral, Dead Band, Speed, and End Points. These parameters are saved to memory on these servos.

It is possible that the servos are not matched because the Speed parameter, for example, for one servo, may have been set differently, at the factory.

There is a Speed setting, in the DX8 programming, for each channel. Perhaps the Speed values are set differently for each channel. Try switching to an Unused Model Memory, Reset it to Factory Defaults, set up the mix between the Elevator channel and the Aux channel you used for the second Elevator half, and see if the Speed is still off on one servo.