Connecting Two Water Storage Tanks

There was a similar post to this one a couple of years back, the difference being that both my tanks are filled from separate branches of the same supply using a ball valve in each tank to stop them filling. I then have an outlet from both tanks connected together that then goes off to a pump. One tank is 5000 litres and the other is 3000 litres, currently the larger of the two is redundant and its outlet valve is off, but come the summer i will need to introduce the second tank. I am located in southern Spain and in the summer they turn off the water supply to save water hence the tanks. My question is as the tanks are of a different size, if i were to fit the smaller tanks outlet with a non return valve so that the larger tank can not fill the smaller one via the outlet pipe, would this work i.e. would the joint outlet pipe still feed the pump correctly or would it have more or less pressure.

I appreciate that i could balance the two tanks, but i would much rather be able to use all the volume i can.

Many thanks.