I now it is not recommended but I need to connect two batteries of 3V in parallel to get extra capacity. Is there a circuit that make any isolation between them but without drop voltage of 0.2V (like when you connect diode)?
There is no such a circuit that will "separate" two connected in parallel batteries without causing voltage drops ..
So, hardwire these two batteries in parallel and that's it ..
The problem is that the two batteries may have different discharge rates. In that case, there would be current from one battery flowing to the other as the higher voltage battery tries to charge the one that depleted more quickly. Likewise, if the batteries are rechargable, you would get different charge currents through each of the batteries - possibly damaging one of them with high temperature because of too high a charging rate.
In vehicles with two batteries, there is a switching circuit that discharges and recharges each of the batteries individually to prevent damage.