I have two different water tanks of 1000 liters each having different source of water to fill the tank.

Now I wanted to install a water pump that can transfer the water from first tank to another or vice versa, this need to be done on a single pipe line and a single water pump, when needed can be used for suction or if needed can be used for delivery.

I have a 1hp water pump that has 1 inlet and 1 outlet, 4 ball valve, many elbow pipe, 1.25 inch upvc pipe, Coupler Pipe, solvent, union pipe, tee pipe

Distance of both the tanks are 100 feet and on same height.

Why not just connect them? If they are the same height, then they will "share" water with no pump needed. Put a shut off in the middle if you need to control when the transfer will or will not happen. This is assuming you can connect to the bottom of each tank.

How often does this need to be done?

If the tanks are at the same level why do you need a pump? How fast does this transfer need to take place?

Get two glasses or small jars and some small flexible tubing.
Sit both glasses on your counter.
Fill one glass 1/2 full of water. Fill the other 1/4 full.
Put tubing in the 1/2 full glass. Suck some water out of the tubing, quickly put your finger over the end of the tubing.
Then insert the tubing in the 1/4 glass. Watch what happens.

Add some water to first one glass and then the other. Watch the water transfer back and forth.

You can do the same thing with your tanks. Run your piping from the bottom of one tank over the top to the top of the other tank and to the bottom. Then fill the pipe with water.

Or you can just run the piping from the bottom of one tank to the bottom of the other.

The larger the piping the faster the transfer.

I thought about the same thing by using a drop into each tank, but at 100 feet, how would he be able, at the start, to suction a 1" (for instance) line over that length?

Then there's always the problem of what to do if the tanks occasionally are empty. You'd have to start over.

Put a hose bib on the line. Connect to house water supply and fill the piping.