Add 2 doubles in string format
If your need is truly


  Add 2 strings representing 2 doubles, return the sum of the 2 doubles in string format.


then it is as easy as

def add_strings(x, y):
    return str(float(x) + float(y))

a = add_strings("2.3", "4.5")

print(a)
print(type(a))


which gives

6.8
&lt;class 'str'&gt;


Note that returning a numerical string is a strange and mostly not required operation in the Python world.
The function should return without the str call, and the conversion to string will either be implicit (like print can do it) or explicit by the user of that function.
If you hand the returned string into a new function that does, say, multiplication, and that function has to convert string to float again... you see the unnecessary chain there.

Other than that, your code can benefit from sticking to the PEP 8 Style Guide.
Most of all, in this case, having all variables in snake_case can save you from hearing "PEP 8"-comments again.
In res = '', you could issue res = "", since those quotes are impossible to mistake.
'' can look like ".