When two resistances are in series, the equivalent resistance is the sum of the resistances: R=R1+R2.

In this case, the same current passes through the two resistances and the potential difference between the limits of the association is the sum of the drop in each resistance: V=V1+V2   

When two resistances are in parallel, the equivalent resistance is calculated as: 1/R=1/R1+1/R2

  In this case the current which comes from the generator is shared in the two branches in an inversely proportional manner to the resistance in each branch: I=I1+I2 while the potential difference is equal in both branches. 

If we associate two identical generators in series, we obtain an emf equivalent to the sum of that of the two batteries; although the internal resistances will also be added up : Total emf = 2·emf-battery  rtotal=2·r

If the two generators are in parallel, the equivalent emf is the same as that of one of them but the internal resistance is reduced to half: rtotal=r/2