Zip them, then sum each tuple.
[sum(x) for x in zip(a,b)]
EDIT : Here's a better, albeit more complex version that allows for weighting.
from itertools import starmap, islice, izip
a = [1, 2, 3]
b = [3, 4, 5]
w = [0.5, 1.5] # weights => a*0.5 + b*1.5
products = [m for m in starmap(lambda i,j:i*j, [y for x in zip(a,b) for y in zip(x,w)])]
sums = [sum(x) for x in izip(*[islice(products, i, None, 2) for i in range(2)])]
print sums # should be [5.0, 7.0, 9.0]
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