How we calcualte the voltage between 2 opposite charged spheres? I know that voltage is the difference in potential energy per charge.
When we have just a single sphere, or some charge, it's intuitive how electrical potential energy varies with distance, maybe because it is analogous to gravitational potential.

As we move the charge/mass against the field, we increase the potential.
But considering the "dual-face" of the electric force what happens when the potentials increase in both directions. Do we add them up?

Given that we have a charge Kq on a sphere and another charge -Zq on another, where Z and K are constants that may not be equal, and the distance between them is d. What is the electrical potential between them?
What is the potential of a negative charge q if we move it away from the positive toward the negative sphere?  
 A: When two charges have opposite sign, they are attracted to each other and the potential energy and voltage between them decrease as they get closer to each other and increase as they move away. This, as you've pointed out, is analogous to gravity.
When two charges have the same sign, they repel each other and the potential energy and voltage between them increase as they get closer to each other and decrease as they move farther away. This is opposite to gravity.
So, when a negative charge moves from a negatively charged sphere to a positively charged sphere, its potential energy decreases in relation to both. 
If the charged spheres were far away from each other, we could calculate the voltage between them, as the sum of potentials of the two spheres at the distance matching to the distance between the spheres. Since the spheres have opposite charges this would result in the addition of the absolute potential values. For instance, if the spheres had the same charge magnitudes, the potential between them would be twice bigger that the potential at that distance created by each individual sphere. 
To calculate those potentials you need to know the charges, the diameters of the spheres and the distance between the spheres and use a standard formula for the potential.
If the spheres are close to each other, assuming that they are conductive, you cannot just add their individual potentials, because as the spheres get close to each other, their charges get redistributed and their individual contributions to the voltage are not the same as they would be if the spheres were alone in space (in which case the charges would be evenly distributed along their surfaces). 
In other words, the principle of superposition, which is applicable to point charges or to conductive spheres at significant distance from each other or to non-conductive charged spheres (since the charges on such spheres don't move), is not directly applicable to the case of conductive spheres close to each other. Naturally, the calculation of the voltage between such  spheres would be more involved.  
