Making the rash assumption that "voltages in series" is to add them...
I'm thinking a 2 input summing amp:
input a = Input 1, input b = output of a 2 input analog switch.
switch input a = ground, input b = Input 2, switch control = output of a comparator.
comparator input a = Input 2, input b = threshold voltage as set by potentiometer.
When Input 2 > threshold, comparator switches & summing amp output = Input 1 + Input 2.
Else: Input 2 <= threshold, summing amp output = Input 1 + 0V (ground)
It should work at small-medium signal levels.
Any kind of real current required will require amplifiers.
High voltages will require scaling then amplifying the result.
If this is a homework assignment, I've already said too much... (bad dog, no biscuit) <<<)))