we are about to install altitude valve between 2 water tanks  the 1st tank is 250 m (elev) and the other one is 90m elev. the valve will be for the lower one , the distance between them is about 2km and the diameter (600mm). The aim of installing this valve is to control the elevation of the tank, to work as pressure reducer and to prevent water hummer. Well it does this proposed work??

The altitude valve installed in line to the lower tank will provide head loss (as you intend) and should cause a proportional amount of water to divert to the other tank. Is it your intent to have equal filling rates to both tanks??  You will have to calculate the proposed loss desired and crank that into the pilot circuitry of the altitude valve.  Be sure to have a level sensing device in each tank to provide feed back, and a to shut off the delivery system (pumps or shut off valves) when the tank(s) reach max level.  The tanks should be vented as well as have overflows.  
The valve if properly set-up can be used to control the water elevation in the lower tank (it can act as a shut off). It will reduce pressue in the line to the lower tank.  I don't see it acting as a water hammer prevention device, except in the speed with which it closes (controlled via needle valves in the pilot lines).

If I guess your intention correctly the altitude valve is intended to give you a better control on the flow to both tanks.  Under normal circumstance the lower tank will get filled first before the higher tank as water always takes the line of the least resistance.

The altitude idea sounds Ok but in practice there may be instability problem with the control mechanism necessitating an expensive valve manufactured for control purposes.

Your system characteristic of the pipeline to the lower tank will be relatively flat and with a small static lift (90m at flow=0, assuming the source of water from elevation 0) but the leg to the higher tank has both higher static lift (250m) and a steep gradient on account of the extra 2km skin friction.  When plotting the two together on the same discharge/head graph the vertical difference between the two is the head loss you need to replicate in the altitude valve to produce equal flow in the system.  It should be obvious that the head loss needed to balance the flows will increase with the discharge rate and continuous flow measurements will have to be installed in at least one half of the system to acquire data for continuously updating the setting of the altitude valve.

If one can control the flow in 50%:50% split one should be able to do it any other combination.

We can leave out the effect of the change of the static lift by the water level moving up the tank as this local effect can be overcome by taking readings on the water level of both tanks to update the two pipeline characteristics dynamically.  Thus theoretically the altitude valve can work satisfactorily.

The water hammer effect is unlikely to be a problem as the altitude valve by its very nature will be slow acting and the valve is controlled as described above.  I could foresee problem only if the control system malfunctions when the valve is allowed to close quicker than the designed rate but this can be mitigated by installing fail save dampers.  Each pipeline has a pressure rating from which the maximum flow change (or the valve closure time) must be observed to avoid damage to the line.

I assume you are currently fill the upper tank with your source and it is currently operating normally, you have a new tank? at the lower area and you want to fill it using the upper tank as its source but not fill it so fast that you lower the pressure in your upper service area.
as such I would recomend an altitude valve with pressure sustaining control loop (also known as a backpressure valve).  The altitude portion of the valve allows it to open if the lower tank is needing water, while the pressure sustaining portion will only allow filling if there is extra water available from the upper tank and the demand in the upper zone is not taking everthing.
There are several manufactorers of pilot controlled diaphram operated control globe valves, use one with a long standing service rep in your area with a good supply of spare parts.  You do not want to wait a week to get a replacement gasket for your valve five years from now.

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