How to configure arduino with voltage sources, V1 and V2 ?

I really struggle to configure arduino to read V1 and V2 with NOT COMMON GROUND?

V1 = .7 to 5 v

Note:
I can read Voltage from in between GND and A1, But if I have 2 voltage source which not shared common ground then how can I measure?

Something like differential voltage.

0788c32207200a14499087f3662fc156649a3dc6.png
Please explain why you cannot have common grounds.

In general, this is not safe, because without a common ground you cannot guarantee that the signals you are applying to the analog inputs are between 0 and 5V with respect to the Arduino’s ground. Your voltage source may be less than 5V with respect to it’s own ground, but that won’t protect the Arduino.

I really struggle to configure arduino to read V1 and V2 with NOT COMMON GROUND?

Not only will you struggle but you will find it impossible unless you use special techniques.

One such is the "flying capacitor" technique, involving a relay that switches both ends of a capacitor between the voltage you are trying to measure and the Arduino. You first switch the capacitor across the voltage source and then change the double pole change over relay to connect it between the analogue input and ground. If you have a higher voltage than 5V you need a potential divider on the Arduino side.